Fix default recipe image fallback to be a valid value: "full"

[saywebsolutions]

Topic and Scope

Fixes issue #1453 and allows the image endpoint to default to returning the full image if no size param is specified.

Concerns/issues

Alternatively the thumb size could be used if it's preferred to default to that instead of full.

Formal requirements

There are some formal requirements that should be satisfied. Please mark those by checking the corresponding box.

• I did check that the app can still be opened and does not throw any browser logs
• I created tests for newly added PHP code (check this if no PHP changes were made)
• I updated the OpenAPI specs and added an entry to the API changelog (check if API was not modified)
• I notified the matrix channel if I introduced an API change

[github-actions]

Test Results

   12 files    588 suites   1m 56s ⏱️
  577 tests   577 ✅ 0 💤 0 ❌
2 308 runs  2 307 ✅ 1 💤 0 ❌

Results for commit b9d25ab.

♻️ This comment has been updated with latest results.