Description of the problem, including code/CLI snippet

Pull request #1110 that has been released in v2.3.0 yesterday introduced a regression on a specific scenario:
when calling len() on the output of Gitlab().projects.list(as_list=False).

Expected Behavior

Being able to call len() on the iterator returned by Gitlab().projects.list(as_list=False)

Actual Behavior

Minimal reproduction code:

import gitlab
len(gitlab.Gitlab(...).projects.list(as_list=False))

Output:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    len(gitlab.Gitlab(...).projects.list(as_list=False))#, pagination='offset'))
  File ".../lib/python3.7/site-packages/gitlab/base.py", line 184, in __len__
    return len(self._list)
  File ".../lib/python3.7/site-packages/gitlab/__init__.py", line 863, in __len__
    return int(self._total)
TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

Workaround:

import gitlab
len(gitlab.Gitlab(...).projects.list(as_list=False, pagination='offset'))

Specifications

• python-gitlab version: sicnce v2.3.0
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): 12.10.7-ee (80b99e43cfc)