Allow removal of global configuration parameters for ImagingService

The conversion to JPG in the Nuxeo Platform is configured with the following contribution (in component org.nuxeo.ecm.platform.picture.ImagingComponent.default.config):

  <extension target="org.nuxeo.ecm.platform.picture.ImagingComponent"
    point="configuration">

    <configuration>
      <parameters>
        <!-- global configuration variables -->
        <parameter name="conversionFormat">jpg</parameter>
      </parameters>
    </configuration>
  </extension>

conversionFormat parameter forces the picture conversion to JPG. In the case we don't want to force a type conversion and just want to keep the original document type, overriding the parameter with an empty string does not do the trick as the service's code sets jpg as a default value if the parameter exists without a value.