[9.x] Add is_string check to QueriesRelationships@requalifyWhereTables

[rodrigopedra]

Closes #42338

This PR:

• Adds a is_string() check before a str_starts_with() on QueriesRelationships@requalifyWhereTables
• Adds a test case which would fail without this patch

Notes:

• The QueriesRelationships@requalifyWhereTables was added by PR [9.x] Add wherehas soft deleting scopes #42100 which tried to fix table prefixing issues when using self-referencing relations
• There is no need to check non-string values, as the idea is to prefix values used in whereColumn, or other that could reference related tables
• The added test case used a new stdClass as enums are not available in PHP 8. As it tests the generated SQL statement, the stdClass instance never acutually gets bound to PDO.

EDIT: added information about added test case. Please read 3rd note about it.

[rodrigopedra]

ping @driesvints , @taylorotwell

P.S.: I didn't add any tests as this is more like a patch bug-fix. I can try adding any later.

EDIT: added test case