Add pack [] case

Author: TeofilC

src/Data/Text/Internal.hs

@@ -243,6 +243,7 @@ int64ToInt32 = fromIntegral
 -- >>> Data.Text.unpack (pack "\55555")
 -- "\65533"
 pack :: String -> Text
+pack [] = empty
 pack xs = runST $ do
   -- Each 'Char' takes up to 4 bytes
   marr <- A.new (length xs `shiftL` 2)
@@ -252,6 +253,6 @@ pack xs = runST $ do
         go (off + d) cs
   len <- go 0 xs
   arr <- A.unsafeFreeze marr
-  return (text arr 0 len)
+  return (Text arr 0 len)
 {-# NOINLINE [0] pack #-}
 -- TODO Do not calculate length xs upfront