Skip empty JARs when includes match no files #510

[AzazelSensei]

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skipIfEmpty only looked at whether the classes directory had any children. A test-jar with includes like **/junit/* still wrote a manifest-only archive when those patterns matched nothing, which is the setup in #510.

I reused the existing FileSet scan so skipIfEmpty also treats "no matching files" as empty. MJAR-162 still skips a missing test-classes directory. MJAR-510 covers the includes case.