Fix tile map loading for collision mask objects

[D8H]

Fixes #4900

• TileMapMask doesn't work with Tiled (1.9.2) exports #4900

The issue was introduced by

• feat: add support for LDtk tilemap #4575

Reproduce steps:

• Open the platformer-with-tilemap example
• Remove all the instance from the scene
• First, add a collision mask object instance to the scene
• Then, add a tile map object instance to the scene
• Save the project.
• Close GDevelop
• Open back GDevelop

Expected:

• The 2 instances show the map.

Actual:

• The 2 instances show a placeholder.

[D8H]

This is the fix, the other changes is just to clean the code a bit.

This method was not adapted when the LDTK handling was added and the Tiled data was returned instead of a tuple.