test_run.py: _custom_fields seems not initialized correctly

[sleviim]

When trying to use the get_custom_field() function for getting a custom field value of a test run, the returned item is None. In test_run.py#get_custom_field there's the following assignment: cf = self._custom_fields Using print(cf) returns an empty list and it seems it wasn't initialized correctly.

Some observations:

• The object seems to be a legit test run (printing tr.test_run_id shows its id number)
• The value seems to be a legit custom field value (I used [print(f.__dict__) for f in tr.get_defined_custom_field_types(project_id="PROJ")] for listing all custom values and used one of them)
• I applied the fix from patch https://github.com/RedHatQE/pylero/pull/151