plugin creates mapping file for empty entry

[elijahiuu]

The plugin will create both JS and CSS entires in wromapping.properties regardless of what is in the wro.xml.

Wro4Mojo.java:126
for (ResourceType resourceType : ResourceType.values()) {..

I end up with a "d41d8cd98f00b204e9800998ecf8427e" hashcodes inside of wromapping when only a CSS or only JS is inside a particular group. Plugin should check if both resource types need to be generated.