If a map value is a message with required fields, the validator should
note that it is uninitialized if a map item contains no value. In this
case, the value is an empty message which obviously does not have the
required field set.
Change-Id: I7698e60765e3c95478f293e121bba3ad7fc88e27
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/213900
Reviewed-by: Joe Tsai <joetsai@google.com>