Clarify the Historical User IDs grammar to include empty localparts

**Link to problem area**: https://spec.matrix.org/v1.4/appendices/#historical-user-ids

**Issue**
The spec should say that empty localparts are acceptable as historical User IDs; e.g. `@:matrix.org` is valid as a User ID (unfortunately) but servers should not allow this to be registered.

This would 'conflict' with

> **It MUST NOT be empty**, and MUST contain only the characters a-z, 0-9, ., _, =, -, and /.

but we already 'conflict' with that when introducing the expanded character set for historical User IDs, so I think this would be more honest and clearer for implementers.

Open the Historical User IDs section at the linked specification page and review the grammar around empty localparts and the existing character-set exception. Clarify that empty localparts such as `@:matrix.org` are valid historically while servers must not allow them to be registered; done when the wording is unambiguous for implementers.

Written by the indexing model from the issue text.