Produce compile-time error on (required) set on an Empty field

[alexander-yevsyukov]

Such an option set on a field with the type google.protobuf.Empty should not compile because enclosing message could not be built using the generated validation code.

When checking for a filled in value, we compare the value with a default instance of a message, and the result would be always true.

This also should include implicitly required field such as IDs for command messages or entity states.