fix(language-core): nullable modelvalues

[davidmatter]

Closes #4646

[johnsoncodehk]

Thanks!

[valh1996]

Hi @johnsoncodehk @davidmatter
Sorry to ping you, I'm using nuxt/ui where modeValue are nullable as suggested by this PR, but I get the following error:

Cannot assign type 'string | null' to type 'string | number | undefined'.
Cannot assign type 'null' to type 'string | number | undefined'

Could this bug persist? Null should be included in the type definition

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VScode version : 1.95.3
Vue Official extension : 2.1.10
Vue version : ^3.5.13
Typescript version : ^5.6.3

[davidmatter]

@valh1996 to improve visibility, would you please create a new issue and link to this PR?