Don't teleport NPCs to unknown cells

Fixes #6521 (closed).

This regression was caused by !1318 (merged) making the workaround introduced in 6a745c01 redundant. Which meant that instead of a no-op being generated, the script suddenly worked. Which is correct, except that only the player should be teleported as a fallback as mentioned in the thread that inspired the initial workaround.

However, Morrowind.exe doesn't accept "" as a cell name (instead complaining that a value must be entered) so to get an unnamed cell (without a region) it allows "Wilderness" to be entered. The other commands also accept it as a valid cell name so now we do as well.