Hamming 2.3.0.10: Add empty strand test cases (#796)

The following solution passes all the current tests but fail on the new ones:

    module Hamming (distance) where

    distance :: String -> String -> Maybe Int
    distance [] []         = Just 0
    distance (x:xs) (y:ys)
      | length(xs) /= length(ys) = Nothing
      | x /= y = fmap (1 + ) (distance xs ys)
      | x == y = distance xs ys

The problem is insufficient pattern matching against an empty strand.

This PR adds two test cases that address the possibility that one strand is
empty and the other one isn't.

This PR addresses exercism/problem-specifications#1450

Author: tqa236

exercises/hamming/package.yaml

@@ -1,5 +1,5 @@
 name: hamming
-version: 2.2.0.9
+version: 2.3.0.10
 
 dependencies:
   - base

exercises/hamming/test/Tests.hs

@@ -61,4 +61,14 @@ cases = [ Case { description = "empty strands"
                , strand2     = "AGTG"
                , expected    = Nothing
                }
+        , Case { description = "disallow left empty strand"
+               , strand1     = ""
+               , strand2     = "G"
+               , expected    = Nothing
+               }
+        , Case { description = "disallow right empty strand"
+               , strand1     = "G"
+               , strand2     = ""
+               , expected    = Nothing
+               }
         ]