CounterTrait now checks for last_update before defaulting to current time.

[jaborsh]

Brief overview of PR changes/additions

• Obligatory Linter PEP adherence.
• CounterTrait calls validate_input after a server reload. Previously, this method would default ._data['last_update'] to the current time making rate freeze until trait referenced. Fix has validate_input check for an existing last update before assigning it the default value time(). [BUG] rate change calculations for traits in the Traits contrib break on reload #3317

[Griatch]

Nice fix, thanks!

Note: This seems to be formatted to 78 character line width. Evennia uses 100 character line width (allowed by PEP8; see CODE_STYLE.md). I'll reformat that after merge, but linting like this makes it much harder to see what the actual functional change is, so best avoid doing so in a PR unless the reformat is the point of said PR :) )