Allow spawner component to be instantiated with an empty list (#78188)

## About The Pull Request

Supplementary to ffd3edc22b

Fixes #78172
The spawner component could be added to arbitrary items by admins but
would always throw an error because it was passed an empty list. Admins
were not capable of providing anything _other_ than an empty list to it
on init, due to limitations in our interface.
We (I but I had help) broke this in #73645 by removing the default
"spawns carp" list.
It is still silly for it to default to spawning carp, but instead it can
now be instantiated with an empty list without breaking, and an admin
can then VV it to further modify the list to what they actually want it
to do.

## Changelog

🆑 
admin: Admins can add/remove the spawner component from arbitrary items
again.
/🆑

code/datums/components/spawner.dm

@@ -38,6 +38,8 @@
 
 /// Try to create a new mob
 /datum/component/spawner/proc/try_spawn_mob()
+	if(!length(spawn_types))
+		return
 	if(!COOLDOWN_FINISHED(src, spawn_delay))
 		return
 	validate_references()