xds: Ensure v2 route match is populated for gRPC

Similar to HTTP, ensure that route match config (which is required by
Envoy) is populated when default values are used.

Because the default matches generated for gRPC contain a single empty
`GRPCRouteMatch`, and that proto does not directly support prefix-based
config, an interpretation of the empty struct is needed to generate the
same output that the `HTTPRouteMatch` is explicitly configured to
provide in internal/mesh/internal/controllers/routes/generate.go.

internal/mesh/internal/controllers/sidecarproxy/builder/routes.go

@@ -473,6 +473,12 @@ func makeGRPCRouteMatch(match *pbmesh.GRPCRouteMatch) *pbproxystate.RouteMatch {
 		default:
 			panic(fmt.Sprintf("unknown method match type: %v", match.Method.Type))
 		}
+	} else {
+		em.PathMatch = &pbproxystate.PathMatch{
+			PathMatch: &pbproxystate.PathMatch_Prefix{
+				Prefix: "/",
+			},
+		}
 	}
 
 	em.HeaderMatches = translateHeaderMatches(match.Headers, nil)