Fix "Detect missions that are missing .start.assign_mission_target.om_special" errors

[BrettDong]

Summary

None

Purpose of change

Detect missions that are missing .start.assign_mission_target.om_special is failing on every pull requests recently: https://github.com/CleverRaven/Cataclysm-DDA/actions/workflows/assign_mission_target_needs_om_special.yml

Fix #70823

Describe the solution

The reason of the error is because #70682 added an overmaps entry that does not have a "overmap" field (the third entry below), causing null errors in jq matching.

{
    "type": "overmap_special",
    "id": "Boat Rental",
    "overmaps": [
      { "point": [ 0, 0, 0 ], "overmap": "boat_rental_north", "locations": [ "lake_shore" ] },
      { "point": [ 0, 0, 1 ], "overmap": "boat_rental_roof_north", "locations": [ "open_air" ] },
      { "point": [ 0, -1, 0 ], "locations": [ "land", "road" ] }
    ]
}

This pull request adds a filter to skip processing overmaps entries without "overmap" field.

Describe alternatives you've considered

Testing

Ran the test locally and it reported no error.

Additional context

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