fix polymorphic serializers for zaken-api

[MatthijsBekendam]

from: https://github.com/VNG-Realisatie/gemma-zaken/issues/2134

Fixed two problems:

• Polymorphic extension grabbed 'discriminator_field' instead of 'group_field'
• fields inside the discriminator which are set equal to 'None' should not be selectable and thus skipped in the 'auto_schema.registry.register_on_missing(linked_component)' step inside the polymorphic extension (polymorphic.py)

[codecov-commenter]

Codecov Report

Base: 46.62% // Head: 52.67% // Increases project coverage by +6.04% :tada:

Coverage data is based on head (2f67a70) compared to base (6a1e675). Patch coverage: 33.33% of modified lines in pull request are covered.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #231 +/- ## ========================================== + Coverage 46.62% 52.67% +6.04% ========================================== Files 66 85 +19 Lines 2181 3434 +1253 Branches 301 472 +171 ========================================== + Hits 1017 1809 +792 - Misses 1122 1540 +418 - Partials 42 85 +43 ``` | [Impacted Files](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie) | Coverage Δ | | |---|---|---| | [...g\_api\_common/extensions/serializers/polymorphic.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vZXh0ZW5zaW9ucy9zZXJpYWxpemVycy9wb2x5bW9ycGhpYy5weQ==) | `0.00% <0.00%> (ø)` | | | [vng\_api\_common/filters.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vZmlsdGVycy5weQ==) | `53.39% <ø> (ø)` | | | [...api\_common/management/commands/generate\_swagger.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vbWFuYWdlbWVudC9jb21tYW5kcy9nZW5lcmF0ZV9zd2FnZ2VyLnB5) | `0.00% <ø> (ø)` | | | [vng\_api\_common/middleware.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vbWlkZGxld2FyZS5weQ==) | `59.44% <ø> (ø)` | | | [vng\_api\_common/mocks.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vbW9ja3MucHk=) | `0.00% <ø> (ø)` | | | [vng\_api\_common/utils.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vdXRpbHMucHk=) | `57.03% <100.00%> (ø)` | | | [vng\_api\_common/api/views.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vYXBpL3ZpZXdzLnB5) | `100.00% <0.00%> (ø)` | | | [vng\_api\_common/constants.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vY29uc3RhbnRzLnB5) | `94.26% <0.00%> (ø)` | | | [vng\_api\_common/api/urls.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vYXBpL3VybHMucHk=) | `100.00% <0.00%> (ø)` | | | [vng\_api\_common/apps.py](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie#diff-dm5nX2FwaV9jb21tb24vYXBwcy5weQ==) | `90.00% <0.00%> (ø)` | | | ... and [15 more](https://codecov.io/gh/VNG-Realisatie/vng-api-common/pull/231?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie) | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=VNG+Realisatie)

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