Fix schema disablement parsing in Studio Extractor

<schema name="files" enabled="false" />

BUILD FAILED - at 2/5/20, 11:43 AM
/home/fox/workspace/nuxeo/packages/src/main/assemble/assembly.xml:121: java.lang.NullPointerException
	at java.base/java.util.Objects.requireNonNull(Objects.java:221)
	at java.base/java.lang.ClassLoader.getResource(ClassLoader.java:1392)
	at org.nuxeo.studio.components.common.runtime.ExtractorContext.getResource(ExtractorContext.java:114)
	at org.nuxeo.studio.components.common.serializer.adapter.schema.SimpleSchemaReader.<init>(SimpleSchemaReader.java:43)
	at org.nuxeo.studio.components.common.serializer.adapter.SchemaAdapter.adapt(SchemaAdapter.java:31)
	at org.nuxeo.studio.components.common.serializer.adapter.SchemaAdapter.adapt(SchemaAdapter.java:26)
	at org.nuxeo.studio.components.common.serializer.JacksonConverter.serialize(JacksonConverter.java:115)
	at org.nuxeo.studio.components.common.serializer.StudioSerializer.serialize(StudioSerializer.java:84)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:195)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1654)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
	at org.nuxeo.studio.components.common.serializer.StudioSerializer.serializeDescriptors(StudioSerializer.java:80)
	at org.nuxeo.studio.components.common.serializer.StudioSerializer.serializeDescriptors(StudioSerializer.java:56)
	at org.nuxeo.studio.components.common.serializer.StudioSerializer.lambda$serializeInto$0(StudioSerializer.java:50)
	at java.base/java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
	at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:658)
	at org.nuxeo.studio.components.common.serializer.StudioSerializer.serializeInto(StudioSerializer.java:50)
	at org.nuxeo.studio.components.common.publisher.Publisher$FilePublisher.publish(Publisher.java:78)
	at org.nuxeo.studio.components.common.ContributionsExtractor.publish(ContributionsExtractor.java:45)
	at org.nuxeo.build.ant.StudioExtraction.execute(StudioExtraction.java:63)
	at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:293)
	at jdk.internal.reflect.GeneratedMethodAccessor25.invoke(Unknown Source)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
	at org.apache.tools.ant.Task.perform(Task.java:348)
	at org.apache.tools.ant.Target.execute(Target.java:435)
	at org.apache.tools.ant.Target.performTasks(Target.java:456)
	at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1405)
	at org.apache.tools.ant.Project.executeTarget(Project.java:1376)
	at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
	at org.nuxeo.build.ant.AntClient.run(AntClient.java:161)
	at org.nuxeo.build.ant.AntClient.run(AntClient.java:119)
	at org.nuxeo.build.maven.AntBuildMojo.execute(AntBuildMojo.java:256)
	at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:210)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:156)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:148)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81)
	at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:56)
	at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
	at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:305)
	at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:192)
	at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:105)
	at org.apache.maven.cli.MavenCli.execute(MavenCli.java:956)
	at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:288)
	at org.apache.maven.cli.MavenCli.main(MavenCli.java:192)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:282)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:225)
	at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:406)
	at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:347)