Fix tests with an empty NPTest.cache file

The NPTest.cache cannot be loaded when empty, and this prevents
getting the data and populating the file. This patch skips the file when
empty as if it didn't exist.

NPTest.pm

@@ -438,7 +438,7 @@ sub LoadCache
 
     chomp($fileContents);
     my( $contentsRef ) = eval $fileContents;
-    %CACHE = %{$contentsRef};
+    %CACHE = %{$contentsRef} if (defined($contentsRef));
 
   }