Handle None in OTEL_PROPAGATORS

[suauk]

Description

Fixes #4143

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• tox -e py312-test-opentelemetry-api
• tox -e py312-test-opentelemetry-sdk
• tox -e mypy

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[xrmx]

@suauk Please add a changelog entry and fix the mypy warning

[suauk]

@suauk Please add a changelog entry and fix the mypy warning

@xrmx Added the entry and resolved mypy warning

[suauk]

@emdneto there's a couple failed checks, both seem to be unrelated to any changes in the PR, both seem to be related to invalid github links, I'm not too sure what to do about those, do you have any guidance?

[emdneto]

@emdneto there's a couple failed checks, both seem to be unrelated to any changes in the PR, both seem to be related to invalid github links, I'm not too sure what to do about those, do you have any guidance?

No idea. Probably a problem with the github itself