Only reference commit script if spending commits in abort

This was failing with PPViewHashesDontMatch because the transaction was
referencing the commit script which is does not always need, and the
commit script being of different version than the other scripts spent.

hydra-tx/src/Hydra/Tx/Abort.hs

@@ -56,7 +56,7 @@ abortTx committedUTxO scriptRegistry vk (headInput, initialHeadOutput) headToken
         unsafeBuildTransaction $
           emptyTxBody
             & addInputs ((headInput, headWitness) : initialInputs <> commitInputs)
-            & addReferenceInputs [initialScriptRef, commitScriptRef, headScriptRef]
+            & addReferenceInputs ([headScriptRef, initialScriptRef] <> [commitScriptRef | not $ null commitInputs])
             & addOutputs reimbursedOutputs
             & burnTokens headTokenScript Burn headTokens
             & addExtraRequiredSigners [verificationKeyHash vk]