Autocompletion: Don't use in operator to decide over variant lookup

[HolonProduction]

Fixes #93799

The in operator is not directly linked to whether get will have success. E.g. for arrays the in operator searches for values while get works with an index.

Instead we can just try to get and only use the result if successful.

[anniryynanen]

I reproduced the issue and confirm that this PR fixes it. The code looks good, I don't think it's likely to break anything.

[akien-mga]

Thanks!