Fix exported property values being lost if base GDScript fails to parse

[neikeq]

Fixes #25121

[neikeq]

There is still an issue:

• Make the base script fail to build/parse.
• Restart the editor (maybe re-opening the scene is enough, I don't know)
• Fix the build/parse error in the base script

The properties are still not showing in the inspector until the derived script is saved again. Nothing is lost though, so at least it's not critical.
Not sure if this is caused by the fallback or not. It doesn't happen in 3.0.0. This stuff is pretty hard to debug :(

[akien-mga]

Thanks!

The properties are still not showing in the inspector until the derived script is saved again. Nothing is lost though, so at least it's not critical.

Might be worth opening an issue about it so that we remember to investigate further, but it's indeed not critical anymore after this fix.

[neikeq]

Done: #25881