[8.x] Fix isRelation() failing to check an Attribute

[rodrigopedra]

If an Eloquent model has a attribute defined using the new Attribute class, Model@isRelation(...) returns true when checking for the attribute name,

This PR:

• Adds a if clause at Model@isRelation(...) to check if a attribute is defined with the same name
• Adds a test that would fail prior to the changes proposed

Note: I am sending this to 8.x as it seems to be a bugfix from the related issue's description.

Closes #40966