fix(ngMock/$httpBackend): fail if a url is provided but is `undefined`

While the `url` parameter is optional for `$httpBackend.when`,
`$httpBackend.expect` and related shortcut methods, it should not have the
value of `undefined` if it has been provided.

This change ensures that an error is thrown in those cases.

Closes #8442
Closes #8462
Closes #10934
Closes #12777

BREAKING CHANGE

It is no longer valid to explicitly pass `undefined` as the `url` argument
to any of the `$httpBackend.when...()` and `$httpBackend.expect...()`
methods.

While this argument is optional, it must have a defined value if it is
provided.

Previously passing an explicit `undefined` value was ignored but this
lead to invalid tests passing unexpectedly.