Verifying maintenance not possible

[stdevel]

Describe the bug Verifying maintenance is currently not possible as katprep_maintenane crashes:

TypeError: object of type 'NoneType' has no len()

To Reproduce Steps to reproduce the behavior:

1. Prepare maintenance
2. Run katprep_maintenance *.json verify
3. See error

Expected behavior Maintenance should be verified.

Screenshots If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• OS version: CentOS 7.5
• Red Hat Satellite or Foreman/Katello version: 1.19.0/3.8.0

Additional context Stacktrace:

$ katprep_maintenance -C pass.auth errata-snapshot-report-xxx.json -I pinkepank verify
Authentication container password:
INFO:katprep_maintenance:This is just a SIMULATION - no changes will be made.
INFO:katprep_maintenance:Snapshot for host 'pinkepank.loc' found.
INFO:katprep_maintenance:Downtime for host 'pinkepank.loc' found.
Traceback (most recent call last):
  File "/home/cstan/.local/bin/katprep_maintenance", line 9, in <module>
    load_entry_point('katprep==0.5.0', 'console_scripts', 'katprep_maintenance')()
  File "/home/cstan/katprep/katprep/maintenance.py", line 885, in cli
    main(options, args)
  File "/home/cstan/katprep/katprep/maintenance.py", line 864, in main
    options.func(options, options.func)
  File "/home/cstan/katprep/katprep/maintenance.py", line 447, in verify
    if len(crit_services) > 0:
TypeError: object of type 'NoneType' has no len()

[okin]

Change proposal: If you make use of Pythons Truth Value Testing the critical code can be simplified:

if len(crit_services) > 0:

Can be written as:

if crit_services:

The length check can be omitted in the same way on most occasions until the length is indeed required. The benefit of this is that a returned None will not result in errors.