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/examples/basic/basic.proto b/encoding/protobuf/examples/basic/basic.proto
index d66aab9..50696ad 100644
--- a/encoding/protobuf/examples/basic/basic.proto
+++ b/encoding/protobuf/examples/basic/basic.proto
@@ -13,4 +13,6 @@
 
     // A method must start with a capital letter.
     repeated string method = 2 [(cue.val) = '[...=~"^[A-Z]"]'];
+
+    map<string, string> exmaple_map = 3;
 }
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