Check emptiness of backlog and str_values to fix tree chopping fetch crash

[ghost]

Summary

SUMMARY: Bugfixes "Check emptiness of backlog and str_values to fix tree chopping fetch crash"

Purpose of change

Fixes #36544

Describe the solution

Check emptiness before trying to access backlog activity or its str values, return empty requirements map if these thigns are empty, activity stops.

Describe alternatives you've considered

N/A

Testing

Stops the crash in the linked issue.

Additional context

I couldnt get the bug to trigger agian by restarting the activity with any of the NPCs there, I cannot think of how the activity str_value had an empty string.
First of all - the NPCs are all outside the range of the usual loot zone search radius, the OP of th eissue said two of the NPCs refused the task and failed correctly.
ONe of them zoomed off to fetch the axe, but didnt store the axe requirement id, which seems impossible from the code - it only applies the fetch activity once its added a string to its string values.

WHenever I tried to get them to do it agian, they alll refused as there wasnt a nearby axe.

But this at least stops it crashing if such a thing occurs again.