jpkg exception

[Jonathan Gibbons]

jpkg should not throw an exception if it is given an empty directory as 
one of its arguments.

In a scenario such as Ant, it is too hard to make the use of an option 
dependent on whether a directory is empty or not.

-- Jon


      [exec] java.io.IOException: Path is empty: 
/w/jjg/work/jigsaw/jigsaw.lt/build/modules/javac/resources
      [exec]     at 
org.openjdk.jigsaw.ModuleFileFormat$Writer.writeFile(ModuleFileFormat.java:233)
      [exec]     at 
org.openjdk.jigsaw.ModuleFileFormat$Writer.writeSection(ModuleFileFormat.java:178)
      [exec]     at 
org.openjdk.jigsaw.ModuleFileFormat$Writer.writeModule(ModuleFileFormat.java:112)
      [exec]     at 
org.openjdk.jigsaw.cli.Packager$Jmod.go(Packager.java:172)
      [exec]     at 
org.openjdk.jigsaw.cli.Packager$Jmod.go(Packager.java:147)
      [exec]     at org.openjdk.jigsaw.cli.Command.run(Command.java:97)
      [exec]     at org.openjdk.jigsaw.cli.Packager.exec(Packager.java:835)
      [exec]     at org.openjdk.jigsaw.cli.Packager.run(Packager.java:628)
      [exec]     at org.openjdk.jigsaw.cli.Packager.main(Packager.java:906)
      [exec] I/O error: Path is empty: 
/w/jjg/work/jigsaw/jigsaw.lt/build/modules/javac/resources
      [exec] Result: 1