[11.x] Where doesnt have nullable morph

[liamduckett]

Hey, this PR aims to resolve this issue.

The problem currently (as described in the issue) is that QueriesRelationships:: excludes Models with a null morph_type value.

This is due to the logic in QueriesRelationships::hasMorph revolving around types (which exludes null):

// src/Illuminate/Database/Eloquent/Concerns/QueriesRelationships.php (231)
if ($types === ['*']) {
    $types = $this->model->newModelQuery()->distinct()->pluck($relation->getMorphType())
        ->filter()
        ->map(fn ($item) => enum_value($item))
        ->all();
}

My proposed solution adds a conditional clause to the query, to also include models with a null morph relation. I made the decision to widen the scope to include other operators - as this felt more complete of a solution than just solving for default whereDoesntHave. I understand this comes with it's own drawbacks, and am happy to partly undo if desired.

This only applies if the chosen type is wildcard, as I understand choosing types currently means any results must have that type of morph relation (and naturally, null cannot acheieve this). Truthfully I'm not particularily confident on this bit - would appreciate any checking over!

I've only added the test case from the issue, as it feels like the majority use case.

Please see my logic for deciding which operators to include below.

operators;
1. >=
  a) positive: doesnt need nulls as is inclusionary
  b) zero:     will always be true...
  c) negative: will always be true...
2. >
  a) positive: doesnt need nulls as is inclusionary
  b) zero:     doesnt need nulls as is inclusionary
  c) negative: will always be true...
3. <=
  a) positive: needs nulls as is exclusionary
  b) zero:     needs nulls as is exclusionary
  c) negative: can never be true...
4. <
  a) positive: needs nulls as is exclusionary
  b) zero:      can never be true...
  c) negative: can never be true...
5. =
  a) positive: doesnt need nulls as is inclusionary
  b) zero:     needs nulls as is exclusionary
  c) negative: can never be true...
6. !=
  a) positive: needs nulls as is exclusionary
  b) zero:     doesnt need nulls as is inclusionary
  c) negative: will always be true...