Influenza: incomplete collection dates default to the first day of the month/year

[joverlee521]

Originally part of #922. Breaking this out into a separate issue after talking with @chaoran-chen because it seems like the issue is resolved in Pathoplexus/Loculus but just not updated here.

Influenza records that have incomplete dates default to the first of the month/year. I'd expect them to keep the incomplete date in the collection date field. Here are a couple examples that I originally flagged in nextstrain/seasonal-flu:

GenBank accession	GenBank date	GenSpectrum accession	GenSpectrum date	Expected date
CY006218	1968	GS_00BVC0L	1968-01-01	1968
MF955471	Mar-1997	GS_00BLBWC	1997-03-01	1997-03

[fhennig]

Thanks for bringing it up! Yes we can keep the incomplete dates.

I'll get to this within the next week I think.

As a note to myself, we can adapt our configs based on this: https://github.com/loculus-project/loculus/blob/main/kubernetes/loculus/values.yaml#L96