fix: check for valid seed before reference in can_do_activity_there()
#4262
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Purpose of change
Fix segfault introduced in ba60edc when attempting to make a farmplot zone of "No Seed" and then trying to farm said plot using automatic actions via "O"->"m".
Describe the solution
Check for valid seed object before referencing it.
Describe alternatives you've considered
Testing
Repro'd the crash based on instructions in bug ticket #4243 on top of 0b5331a by following ticket repro steps. Tested fix by following same steps and seeing that character can now plow the farm field just fine.
Additional context
Checklist