Generate empty source values 20% of the time.

Co-Authored-By: Patrick Thomson <patrickt@users.noreply.github.com>

Author: robrix

test/Generators.hs

@@ -9,4 +9,6 @@ import qualified Data.Source
 import Data.Functor.Identity
 
 source :: (GenBase m ~ Identity, MonadGen m) => Hedgehog.Range Int -> m Data.Source.Source
-source r = Data.Source.fromUTF8 <$> Gen.utf8 r (Gen.frequency [ (1, pure '\r'), (1, pure '\n'), (20, Gen.unicode) ])
+source r = Gen.frequency [ (1, empty), (20, nonEmpty) ]
+  where empty    = pure mempty
+        nonEmpty = Data.Source.fromUTF8 <$> Gen.utf8 r (Gen.frequency [ (1, pure '\r'), (1, pure '\n'), (20, Gen.unicode) ])