Handle missing qgis_projects_output_dir configuration

[pka]

DataServiceConfig raises an exception when qgis_projects_output_dir is missing:

Traceback (most recent call last):
  File "config_generator_cli.py", line 72, in <module>
    generator.write_configs()
  File "/home/pi/code/gis/qwc-services/qwc-config-generator/config_generator/config_generator.py", line 252, in write_configs
    self.write_service_config(service_config['name'])
  File "/home/pi/code/gis/qwc-services/qwc-config-generator/config_generator/config_generator.py", line 279, in write_service_config
    config = config_handler.config()
  File "/home/pi/code/gis/qwc-services/qwc-config-generator/config_generator/data_service_config.py", line 50, in config
    resources['datasets'] = self._datasets(config, session)
  File "/home/pi/code/gis/qwc-services/qwc-config-generator/config_generator/data_service_config.py", line 92, in _datasets
    if qgs_reader.read(qgs_name):
  File "/home/pi/code/gis/qwc-services/qwc-config-generator/config_generator/qgs_reader.py", line 31, in read
    qgs_path = os.path.join(self.qgs_resources_path, qgs_file)
  File "/usr/lib/python3.8/posixpath.py", line 76, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType