[FAU-434] Include campo keys from all related terms to degree program data

[zhyian]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Feature

What is the current behavior? (You can also link to an open issue here)
https://inpsyde.atlassian.net/browse/FAU-434

1. If a degree program has multiple terms with different campo keys, we only pass the keys from the last term(code).

2. if the specified campo keys don't match any terms, the tax_query is omitted entirely, causing the query to return all posts, which is unintended.

What is the new behavior (if this is a feature change)?

1. All related term campo keys are now included in the degree program data.
2. Instead of omitting the tax_query, we now pass 0 as required term ID, which ensures no posts are returned, as there is no term with ID 0.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
No

Other information:
RRZE-Webteam/FAU-Studium-Embed#42

[tyrann0us]

Thank you for working on this! Code-wise LGTM. 💪🏽
Please forgive the silly question, but since we change the schema, does this require migration of existing data?

[zhyian]

Thanks for the question. I believe all that we need it's just to invalidate cache on the providing website.

[amiut]

I think we need to update again the changes made in this PR RRZE-Webteam/FAU-Studium#144, now that the campo keys format is updated, right?

[amiut]

LGTM, thanks a lot

[shvlv]

Thanks!