encoding/protobuf: make map field as not required by default

in proto3 all fields are optional, thus making the map field optional as well.

Change-Id: I5397adf6bfc990d6af248acef5bc4dcdf89c5cdd
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2981
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/protobuf/testdata/client_config.proto.out.cue b/encoding/protobuf/testdata/client_config.proto.out.cue
index 527eead..50d61d5 100644
--- a/encoding/protobuf/testdata/client_config.proto.out.cue
+++ b/encoding/protobuf/testdata/client_config.proto.out.cue
@@ -144,7 +144,7 @@
 	// Map of control configuration indexed by destination.service. This
 	// is used to support per-service configuration for cases where a
 	// mixerclient serves multiple services.
-	serviceConfigs: {
+	serviceConfigs?: {
 		<_>: ServiceConfig
 	} @protobuf(2,type=map<string,ServiceConfig>,service_configs)