[9.x] Add havingNull to query builder

[amir9480]

This PR adds havingNull and havingNotNull support to the query builder class.
The syntax is exactly like whereNull and whereNotNull.

Example code before this change:

User::select(...)
    ->join(...)
    ->groupBy(...)
    ->havingRaw('users.sample_column is null')
    ->get();

Same code after this change:

User::select(...)
    ->join(...)
    ->groupBy(...)
    ->havingNull('sample_column')
    ->get();