Prevent deploy of jar to Maven repo if only contains a MANIFEST.MF file

[plaird]

In PR #84 we had a case where a user was trying to deploy a java_binary rule jar to the Maven repo. But the CI job was not running the build for the :* target, so the xyz_deploy.jar was missing so the logic picked the xyz.jar instead. For java_binary rules, that jar is empty except for a MANIFEST.MF file. Since such a jar is never(?) useful, we should fail on that case.

https://github.com/salesforce/pomgen/blob/master/maven/maven_functions.sh#L231