fix: member not being cached in Guild.getch_members in the case of user_ids of length one (#974)

## Summary

`Guild.get_or_fetch_members` uses `Guild.fetch_member` in case
`user_ids` contains only 1 unresolved ID. However, it does not cache the
value afterwards, which is certainly a bug that we missed.

## Checklist

<!-- Put an x inside [ ] to check it, like so: [x] -->

- [x] If code changes were made, then they have been tested
    - [ ] I have updated the documentation to reflect the changes
    - [x] I have formatted the code properly by running `pdm lint`
    - [x] I have type-checked the code by running `pdm pyright`
- [x] This PR fixes an issue
- [ ] This PR adds something new (e.g. new method or parameters)
- [ ] This PR is a breaking change (e.g. methods or parameters
removed/renamed)
- [ ] This PR is **not** a code change (e.g. documentation, README, ...)

Author: EQUENOS

changelog/974.bugfix.rst

@@ -0,0 +1 @@
+Fix :meth:`Guild.get_or_fetch_members` not caching anything in the case of 1 unresolved ID.

disnake/guild.py

@@ -4294,7 +4294,10 @@ async def get_or_fetch_members(
         if len(unresolved_ids) == 1:
             # fetch_member is cheaper than query_members
             try:
-                members.append(await self.fetch_member(unresolved_ids[0]))
+                member = await self.fetch_member(unresolved_ids[0])
+                members.append(member)
+                if cache:
+                    self._add_member(member)
             except HTTPException:
                 pass
         else: