Fix Explorer operation pages on 5.5

FTL stack trace ("~" means nesting-related):
	- Failed at: ${Root.currentDistribution.javaDocHel...  [in template "docMacros.ftl" in macro "javadoc" at line 32, column 14]
	- Reached through: @javadoc nxItem.operationClass, true,...  [in template "file:/opt/apidoc/nuxeo-server-tomcat-11.3.56/nxserver/web/root.war/modules/org.nuxeo.apidoc.webengine/skin/views/operation/default.ftl" at line 70, column 11]
	- Reached through: @block name="right"  [in template "file:/opt/apidoc/nuxeo-server-tomcat-11.3.56/nxserver/web/root.war/modules/org.nuxeo.apidoc.webengine/skin/views/operation/default.ftl" at line 4, column 1]
	- Reached through: @extends src="base.ftl"  [in template "file:/opt/apidoc/nuxeo-server-tomcat-11.3.56/nxserver/web/root.war/modules/org.nuxeo.apidoc.webengine/skin/views/operation/default.ftl" at line 1, column 1]
----
	at org.nuxeo.ecm.platform.rendering.fm.FreemarkerEngine.render(FreemarkerEngine.java:193)
	at org.nuxeo.ecm.webengine.model.impl.AbstractWebContext.render(AbstractWebContext.java:641)
	... 94 more
Caused by: freemarker.core._TemplateModelException: No compatible overloaded variation was found; declared parameter types and argument value types mismatch.
The FTL type of the argument values were: Null.
The Java type of the argument values were: Null.
The matching overload was searched among these members:
    org.nuxeo.apidoc.documentation.JavaDocHelper.getUrl(String, String),
    org.nuxeo.apidoc.documentation.JavaDocHelper.getUrl(String)