Gracefuly handle missing values in POST oai-pmh/enrichedInstances

Overview:

When requested instance has corresponding holdings record with missing electronicAccess.resourceId the request returns 400 status code

Steps to Reproduce:

1. Identify the instance that has holdings associated with it.  Electronic access has to be populated but relationshipId will need to be empty
2. send POST request to oai-pmh/enrichedInstances

Expected Results:

200 status code, response contains available instance , holdings and items data
Actual Results:

400 status code
Additional Information:
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