Fix amigara horror not spawning.

Fixes #1797

The commit 4b7038a7a8d9b3eff87ce081474160104ffddde9 lost the `!` for some reasons.

The loop is supposed to stop when the chosen place *is empty*. In other words, continue (to search for a valid spawn point) while *it's not empty*. The current version left the loop only because it reached 10 tries, and in this case the monster would not spawn, or it left the loop when the chosen square was not empty because the player was already there.