Null values in configuration cause issues

[gouttegd]

FYI:

seed without a config file, cd into src/ontology, run odk update:

File ".../config.py", line 242, in update_config_dict
    for imp in obj["import_group"]["products"]:
TypeError: 'NoneType' object is not subscriptable

Originally posted by @matentzn in #26 (review)

[gouttegd]

The issue above is due to the fact that, when seeding without a configuration file, the automatically generated configuration file is full of null values:

babelon_translation_group: null
bridge_group: null
components: null
config_hash: null
contact: null
contributors: null
creators: null
description: null
documentation: null
import_group: null
namespaces: null
pattern_pipelines_group: null
public_release: null
public_release_assets: null
robot:
  obo_format_options: --clean-obo "strict drop-untranslatable-axioms"
  plugins: null
  […]
sssom_mappingset_group: null
subset_group: null

The issue specifically reported by @matentzn is due to import_group: null and would be trivially fixable by adding an ad-hoc null check in line 242 of config.py, but

(1) I am concerned we could end up “playing catch-up” with the same kind of bugs for a long time, and

(2) Those null values are in fact polluting the config file needlessly.

Therefore I think a cleaner and more future-proof fix is to make sure those null values are simply removed from the config file altogether.

[matentzn]

I am fine with it that - but note that I would even be fine with simply deprecating the option of generating a config.