Validation error for Sample pid field when ICAT value is None

[VKTB]

Description:
Sending the following request:
http://{{datagateway-api}}/search-api/datasets?filter={"include": [{"relation": "samples"}]}

returns 500 INTERNAL SERVER ERROR:

pydantic.error_wrappers.ValidationError: 1 validation error for Sample
    pid
    field required (type=value_error.missing)

For ISIS, the Sample pid PaNOSC field maps to the Sample pid field in ICAT, however the pid values of some ICAT samples are None. These samples are the ones that are causing ValidationError because the Sample pid PaNOSC field is defined as mandatory in the Sample pydantic model which means that it does not accept a None value.

Following a chat with @louise-davies, we decided that we should set the pid at the Search API level. For example, when pid of an ICAT sample is None, we should instead take its id and set the PaNOSC pid to be pid:<id>. The decision was made based on the suggestions in icatproject/icat.server#231

Acceptance criteria:

• Datasets with Samples should be returned

[VKTB]

We will probably face the same issue highlighted in #308 if we are setting the pid at the Search API level.