feat(ext/ffi): support null in buffer argument

[DjDeveloperr]

• Adds support for null in buffer argument. Corresponds to nullptr in FFI.
• Adds relevant test case.

Fixes #12809

[bartlomieju]

@DjDeveloperr tentatively this looks good to me, but please update type declarations to reflect that if buffer is null then buffer length shouldn't be passed as the next argument. Also an update to manual would be appreciated too.

[DjDeveloperr]

Do I add that to type declarations of Deno.ForeignFunction's parameters field? I didn't quite get as passing length of buffer as next argument is not required.

[bartlomieju]

Do I add that to type declarations of Deno.ForeignFunction's parameters field? I didn't quite get as passing length of buffer as next argument is not required.

I guess you don't need to add any types to ForeignFunction, but I'd be good to add some examples into the JSDoc for it.

[DjDeveloperr]

@bartlomieju Added an example for buffer argument and some notes about null buffer argument in JS doc.

[DjDeveloperr]

Looks like #12828 adds this too.. I guess I'll close this.