Restore shrinking behaviour for TokenQuantity in TokenMapSpec.

The shrinker for `Arbitrary TokenQuantity` should allow zero values.

See:

#2765 (review)

lib/core/test/unit/Cardano/Wallet/Primitive/Types/TokenMapSpec.hs

@@ -1050,7 +1050,7 @@ instance Arbitrary TokenQuantity where
     -- verify that the token map invariant (that a map contains no
     -- zero-valued tokens) is maintained.
     arbitrary = genTokenQuantitySmall
-    shrink = shrinkTokenQuantitySmallPositive
+    shrink = shrinkTokenQuantitySmall
 
 instance Arbitrary (Positive TokenQuantity) where
     arbitrary = Positive <$> genTokenQuantitySmallPositive