CSSMin: Clean up the logic in getLocalFileReferences()

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