rake-compiler JavaExtensionTask should provide sanity check when there is no compilation

[myabc]

To reproduce, create a basic project structure.

mkdir ext/my_java_extension

And create a blank .java file (MyJavaExtensionService.java) in the above directory.

Create a Rakefile with the following:

Rake::JavaExtensionTask.new('my_java_extension')

javac will not complain about being passed a blank .java file, but will not emit a .class file. jar will then be passed zero file arguments and output an error/usage message.