sweep: make sure nil tx is handled

After previous commit, it should be clear that the tx may be failed to
created in a `TxFailed` event. We now make sure to catch it to avoid
panic.

sweep/fee_bumper_test.go

@@ -91,13 +91,6 @@ func TestBumpResultValidate(t *testing.T) {
 	}
 	require.ErrorIs(t, b.Validate(), ErrInvalidBumpResult)
 
-	// A failed event without a tx will give an error.
-	b = BumpResult{
-		Event: TxFailed,
-		Err:   errDummy,
-	}
-	require.ErrorIs(t, b.Validate(), ErrInvalidBumpResult)
-
 	// A fatal event without a failure reason will give an error.
 	b = BumpResult{
 		Event: TxFailed,
@@ -118,6 +111,13 @@ func TestBumpResultValidate(t *testing.T) {
 	}
 	require.NoError(t, b.Validate())
 
+	// Tx is allowed to be nil in a TxFailed event.
+	b = BumpResult{
+		Event: TxFailed,
+		Err:   errDummy,
+	}
+	require.NoError(t, b.Validate())
+
 	// Tx is allowed to be nil in a TxFatal event.
 	b = BumpResult{
 		Event: TxFatal,