MiniMaven: allow building .jar files without Java sources

Now you can build pure-resources .jar files, too, by simply omitting
src/main/java/ and only providing src/main/resources/.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a rudimentary test for MiniMaven

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>