DPL-nnn Decide what to do when Null values for testing date

[emrojo]

User story
As PSD/GSU we want to understand what should we do when the samples we receive don't have a testing date defined. At now we are applying a default of 1/1/1970 to these samples. If we want to support them, we should update the schema to allow nulls, and perform checks in the project as well

Who are the primary contacts for this story
Prasanth (GSU)
Eduardo (PSD)

Who is the nominated tester for UAT
e.g. John S (don't include surnames in public repos)

Acceptance criteria
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Dependencies
This story is blocked by the following dependencies:

• #<issue_no.>
• sanger/#<issue_no.>

References
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• sanger/#<issue_no.>

Additional context
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