[lexical-selection] Fix __language in null

[ivailop7]

5th highest error we are observing is: 'Cannot use 'in' operator to search for __language in null'
Not being able to reproduce, but confident the below should address this error in particular, since it seems like the only place in the code base we call '__language'

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
lexical	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 20, 2024 10:33pm
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[github-actions]

size-limit report 📦

Path	Size
lexical - cjs	29.94 KB (0%)
lexical - esm	29.78 KB (0%)
@lexical/rich-text - cjs	38.51 KB (0%)
@lexical/rich-text - esm	31.58 KB (0%)
@lexical/plain-text - cjs	37.16 KB (0%)
@lexical/plain-text - esm	29 KB (0%)
@lexical/react - cjs	40.29 KB (0%)
@lexical/react - esm	33 KB (0%)

[etrepum]

#6701 has a repro, this will certainly fix this exception but I think another one might happen because other invariants are being violated when this occurs #6701 (comment)

[etrepum]

Approving because it's an incremental improvement but I think this may just raise an exception elsewhere based on previous comment

[ivailop7]

I'll rebase to latest nightly and see where things are at, before merging this one. Will keep open for a week/two.

[ivailop7]

After rebasing to latest it's been 7 days without observing the error. This won't be needed it appears. Closing.