Fixes Odyssey maps spawning outside of its gamemode

[kano-dot]

To break down the way we handle map spawning logic:

• in mapsystem/map.dm, build_away_sites() evaluates every map_template and organizes suitable candidates considering: flags they bear, if the current sector allows this away site to spawn, etc...

• It randomly picks among available maps via pickweight(), decreasing relevant cost from the total budget

So far why this never caused any problems is, pickweight() will never pick a key with 0 value unless every key passed to this function has 0 value. What was happening here is system runs out of other candidates before exhausting its budget, leaving only odyssey maps with 0 weight in the list.

This fix should only affect natural map spawning process, since Odyssey has its own method on choosing candidates. Fixes #21276

[kano-dot]

!bugfix

[kano-dot]

!review