[11.x] Fix docblock for collection pluck methods

[SanderMuller]

Arr::pluck() allows passing null as the $value, however src/Illuminate/Collections/Collection::pluck() and src/Illuminate/Database/Eloquent/Collection::pluck() don't include null int he docblock for the $value param.

This causes static analysis to report that passing null as the value is not allowed, e.g.:

  304    Parameter #1 $value of method                                                          
         Illuminate\Database\Eloquent\Collection<int,Illuminate\Database\Eloquent\Model>::pluck()  
         expects array<string>|string, null given.

See https://github.com/laravel/framework/blob/11.x/src/Illuminate/Collections/Arr.php#L545 for the Arr::pluck() definition containing null