backend: document what happens to missing keys and empty fields in extract-unprocessed-data

[corneliusroemer]

loculus/backend/src/main/kotlin/org/loculus/backend/api/SubmissionTypes.kt

Line 143 in 1231abd

description = "The key is the segment name, the value is the nucleotide sequence",

[corneliusroemer]

Per the prepro specs it should never be null - so we should test that empty fields are indeed returned as "" rather than null

[chaoran-chen]

I believe that it should never by null or "" but instead mapped to main. And that's just "internally" within Loculus/LAPIS/pipeline but won't be ever exposed to the end user. For the pipeline, the use of main is documented here (but maybe too briefly).

[corneliusroemer]

Ah sorry I referenced the wrong column, we were wondering about the metadata value type - the spec is good, but swagger should have the same info and is currently not quite as explicit as it could be