Add test to as_interface when no cls or methods

Add test to as_interface when no cls or methods

Fixes #4511

### Description
The `as_interface` function requires that an argument be provided for at least one of the parameters `cls` or `methods`. There was no test that tested the case when neither of these arguments were provided.

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #4512
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4512
Pull-request-sha: 156f6af95ede730d2e1a9d2c935c830cf3cfc96e

Change-Id: Ifa23dbe1dbe9a824fe680e4947b81c96a50c133f