CSSMin: Clean up the logic in getLocalFileReferences()

The function used to check if the $path parameter is null on every
inner loop iteration, only to effectively return an empty array if it
was. Rephrased the code to say exactly that.

(Not sure what the behavior is good for, though…)

Change-Id: I716bfde581d3ed4c02f0d9f544beb9d0a1906261
Merge "CSSMin: Clean up the logic in getLocalFileReferences()"