Adds nullability check to HasOperations method and updates/refactors tests for OpenApiUrlTreeNode

[irvinesunday]

This PR:

• Updates the XML documentation for the HasOperations() method in the OpenApiUrlTreeNode.cs class.
• Adds a nullability check for the label param in the above method.
• Update tests for the above method to validate the nullability check.
• [Bonus] Refactors out common code in the OpenApiUrlTreeNodeTests.cs into reusable private properties, and sorts all ArgumentExceptions tests to come after the rest of the other tests. This is to help in test organization.
• [Bonus] Adds further tests to the ArgumentExceptions tests for Create() and Attach() methods.