Slightly increase the probability of generating empty UTxO indices.

This change alters the `genUTxOIndexSmall` generator to slightly
increase the probability of generating empty and singleton UTxO indices.

This is useful because it allows greater coverage of boundary conditions
that occur only when indices are very small.

lib/core/src/Cardano/Wallet/Primitive/Types/UTxOIndex/Gen.hs

@@ -18,15 +18,19 @@ import Cardano.Wallet.Primitive.Types.UTxOIndex
 import Control.Monad
     ( replicateM )
 import Test.QuickCheck
-    ( Gen, choose, shrinkList )
+    ( Gen, choose, frequency, shrinkList )
 import Test.QuickCheck.Extra
     ( shrinkInterleaved )
 
 import qualified Cardano.Wallet.Primitive.Types.UTxOIndex as UTxOIndex
 
 genUTxOIndexSmall :: Gen UTxOIndex
 genUTxOIndexSmall = do
-    entryCount <- choose (0, 64)
+    entryCount <- frequency
+        [ (1, pure 0)
+        , (1, pure 1)
+        , (32, choose (2, 64))
+        ]
     UTxOIndex.fromSequence <$> replicateM entryCount genEntrySmallRange
 
 shrinkUTxOIndexSmall :: UTxOIndex -> [UTxOIndex]