Copied this feature from prost. A RPC library could use nimpb_build to generate
RPC code automatically based on service definitions in .proto files.

When a message definition doesnt have any fields, it could be that in the future
the definition has fields. The message on the wire could have unknown fields,
even if our definition does not have any fields. So, we should read fields
for "empty" messages and store the unknown fields so that we can pass them
through if needed.

The compiler crashed because it didn't have a default value for an enum defined
in another file. Now the compiler finds the definition of the enum when asked
for a default value of an enum.