keep Editor in local hosted mission browser while hiding stock missions

[commy2]

When merged this pull request will:

• Currently, in local hosted MP only, you can select a special editor "mission" to open MP 3den. In MP 3den when using the preview the lobby opens and connected clients can pick slots. Very helpful for debugging btw.
• With the addition of the stock mission filter when filtering stock missions this entry will be hidden from the list, despite not being a stock mission technically.
• Since a way to open the editor is pretty much the opposite of using a stock mission, I want to keep this entry even when the filter is active.
• The reason it becomes hidden is because the list box entry has value "" instead of "missionName.mapName". "" splitString "." is the empty array. [] select 0 is nil, no error. ARRAY find nil is nil and thus the if executes neither the then nor the else block. Basically something that errors in scheduled environment.
• switching to param with default value makes my script recognize the mission name of the editor "mission" as "" instead of undefined, and thus it is correctly recognized as not a stock mission.