Asset Pipeline doesn't handle a missing file or src attribute in a g.resource taglib call

[Simon Bear]

The regular grails tags allows for g.resource to omit the file or src attribute in order to reference a directory under web-app. With the asset pipeline, this will cause a Null Pointer Exception in AssetProcessorService.getResolvedAssetPath(final String path, final ConfigObject conf = grailsApplication.config.grails.assets)