FileCacheStore: skip unnecessary stream creation if file is empty

The method FileCacheStore.loadBucket() does not need to call "new FileInputStream()", when the file is empty (length==0).
So it might be better to add the following lines before the call to "new FileInputStream(bucketFile)":

// BEGIN: new stuff
if (bucketFile.length()==0)
return null;
// END: new stuff