Is group info message required (spec says no) or not (implementation returns error when not present)?

**Describe the bug**
I got a pull request for review the the PureHDF repo (https://github.com/Apollo3zehn/PureHDF/pull/178) which wants PureHDF to emit a group info message, otherwise the C library is unable to modify files created by PureHDF. The "bug" here is that the spec says that the group info message is optional:

I accepted the pull request to stay compatible with the C library but it would be nice to clarify the wording in the spec or, alternatively update the C library to gracefully fall back to default group info values when that message is not present.

Link to source code where the error is thrown: https://github.com/HDFGroup/hdf5/blob/develop/src/H5Gobj.c#L465-L466

**Expected behavior**
C library should not throw `message type not found` errors when the optional group info message is not present or the specs wording changes to reflect the fact that the group info message is required.

**Platform (please complete the following information)**
- HDF5 version: f8f0379c7c5ef3cd4939cb9e115d2fd3502e1748