fix: Correct check if a requirement is a direct requirement

[johbo]

"r.comes_from" can be "None" if a requirement is directly specified and not listed in a requirements file.

I've noticed the following traceback:

ERROR: Exception:
Traceback (most recent call last):
  File "/nix/store/r3g6kc8h7li85wbsnnw07053kghc8wb2-python3.10-pip-20.1.1/lib/python3.10/site-packages/pip/_internal/cli/base_command.py", line 188, in _main
    status = self.run(options, args)
  File "/nix/store/kvqpw2w6ck254qp99jn84y66yr3sg2nv-python3.10-pip2nix-0.9.0.dev1/lib/python3.10/site-packages/pip2nix/generate.py", line 343, in run
    with Contexter() as ctx:
  File "/nix/store/gda35s57qcxrmmfj28qw7n1izqs1zc25-python3.10-contexter-0.1.4/lib/python3.10/site-packages/contexter.py", line 96, in __exit__
    reraise(exc)
  File "/nix/store/gda35s57qcxrmmfj28qw7n1izqs1zc25-python3.10-contexter-0.1.4/lib/python3.10/site-packages/contexter.py", line 26, in reraise
    raise exc[1].with_traceback(exc[2])
  File "/nix/store/kvqpw2w6ck254qp99jn84y66yr3sg2nv-python3.10-pip2nix-0.9.0.dev1/lib/python3.10/site-packages/pip2nix/generate.py", line 346, in run
    requirement_set = self.super_run(options, args)
  File "/nix/store/kvqpw2w6ck254qp99jn84y66yr3sg2nv-python3.10-pip2nix-0.9.0.dev1/lib/python3.10/site-packages/pip2nix/generate.py", line 325, in super_run
    self.process_requirements(
  File "/nix/store/kvqpw2w6ck254qp99jn84y66yr3sg2nv-python3.10-pip2nix-0.9.0.dev1/lib/python3.10/site-packages/pip2nix/generate.py", line 88, in process_requirements
    packages_base = [
  File "/nix/store/kvqpw2w6ck254qp99jn84y66yr3sg2nv-python3.10-pip2nix-0.9.0.dev1/lib/python3.10/site-packages/pip2nix/generate.py", line 90, in <listcomp>
    if r.is_direct and not r.comes_from.startswith('-c ')]
AttributeError: 'NoneType' object has no attribute 'startswith'

Found the issue while running the following command:

pip2nix generate trytond