graph/db: let FetchChannelEdgesByID behave as promised

The comment of FetchChannelEdgesByID says that if the ErrZombieEdge
error is returned, then the ChannelEdgeInfo return parameter will also
still be populated and returned (ie, wont be nil). This commit updates
the SQLStore implementation of FetchChannelEdgesByID to do this. This is
needed to prevent nil dereference panics at any call-sites that depend
on the method working as it describes.

graph/db/graph_test.go

@@ -451,8 +451,14 @@ func TestEdgeInsertionDeletion(t *testing.T) {
 	// properly deleted.
 	_, _, _, err = graph.FetchChannelEdgesByOutpoint(&outpoint)
 	require.ErrorIs(t, err, ErrEdgeNotFound)
-	_, _, _, err = graph.FetchChannelEdgesByID(chanID)
+
+	// Assert that if the edge is a zombie, then FetchChannelEdgesByID
+	// still returns a populated models.ChannelEdgeInfo as its comment
+	// description promises.
+	edge, _, _, err := graph.FetchChannelEdgesByID(chanID)
 	require.ErrorIs(t, err, ErrZombieEdge)
+	require.NotNil(t, edge)
+
 	isZombie, _, _, err := graph.IsZombieEdge(chanID)
 	require.NoError(t, err)
 	require.True(t, isZombie)