Fixing directory creation to not create "" directories inside hpc report (harmless but needless)

HpcMarkup.hs

@@ -17,6 +17,7 @@ import Data.List
 import Data.Maybe(fromJust)
 import Data.Array
 import Data.Monoid
+import Control.Monad
 import qualified HpcSet as Set
 
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@@ -453,7 +454,8 @@ writeFileUsing filename text = do
 
 #if __GLASGOW_HASKELL__ >= 604 
   -- create the dest_dir if needed
-  createDirectoryIfMissing True dest_dir
+  when (not (null dest_dir)) $
+    createDirectoryIfMissing True dest_dir
 #endif
 
   writeFile filename text