Parameter to skip the generation of the sources jar

Generation of sources-jar should be automatically skipped when no sources were found.

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Just in the same way as javadoc-plugin does!
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For example if the sources- (default: src/main/java, configured value should be taken into account) and testsources-folder are missing then no sources-jar should be generated. Also if the folders are there but no files in that folders are available the generation should be skipped.

In other words: don't generate a sources jar which contains only META-INF/MANIFEST.MF with following content:
Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Created-By: 11.0-b15 (Sun Microsystems Inc.)

Reason:

• Generated sources-jar for empty sources is useless
• Generated sources-jar for empty sources spams the maven repository!
• Generation of sources-jar "costs" expensive build time
• explicit skipping of plugin (which is one of the features for 2.2-release) is annoying/would be an expensive work as you have to go through all the pom's...

If you are aware of old behaviour (someone could need the useless empty jars) then provide the possibility to configure/enable the automatic skip behaviour.