fix introducing unmapped chars into keymap

fix the keymap bug #4532

Bug can be reproduced like this:
choose a key that is not mapped to any action,
start the waiting action,
during waiting press that unmapped key,
go into the command mapping menu (? -> 1),
change something to there to trigger storing the command map,
look at data/keymap.txt

Bug is using operator[] on keymap which inserts the key if that key is not present in the map, using the default constructed value (which is ACTION_NULL).
This will insert the ACTION_NULL into the keymap and therefor save a keymap with "null" as action string.