Extensions for primitive type 'code' cannot be uploaded individually.

[welbergm-draeger]

Describe the bug
Extensions should be uploadable without their corresponding 'origin' attributes.
Example 1:

{
    "resourceType": "Patient",
	"active": false,
	"_gender": {
        "extension": [{
            "url": "some.url.com",
            "valueCode": "undefined"
        }]
    }
}

POSTing the resource of Example 1 produces the following OperationOutcome issue:

Response 1:

"issue": [
        {
            "severity": "error",
            "code": "invalid",
            "diagnostics": "Value cannot be null.  Parameter name: Patient.gender | Person.gender | Practitioner.gender | RelatedPerson.gender."
        }
    ]

This problem appears to only affect attributes of the type 'code'.

FHIR Version?
R4

Data provider?
CosmosDB

To Reproduce
Steps to reproduce the behavior:

1. POST patient resource from Example 1 to the FHIR server.

Expected behavior
POSTing the patient resource from Example 1 should not lead to an error, but should create the resource successfully instead.

Resources containing only extension attributes of types other than 'code' (e.g. 'date') produce the expected behaviour already:
Example 2:

{
    "resourceType": "Patient",
	"active": false,
	"_birthDate": {
        "extension": [{
            "url": "some.url.com",
            "valueString": "undefined"
        }]
    }
}

Uploading resources with extensions for 'code' type attributes already works, when the 'origin' attribute is also present.
Example 3:

{
    "resourceType": "Patient",
	"active": false,
	"gender": "male",
	"_gender": {
        "extension": [{
            "url": "some.url.com",
            "valueCode": "undefined"
        }]
    }
}

Example 1 should produce the same response as Examples 2 and 3 (e.g. a valid resource is created on the server),
as "The extensions may appear in place of the value of the primitive datatype." [1].

Actual behavior
POSTing the patient resource from Example 1 to the azure FHIR server leads to a HTTP 400 Bad Request server response.
Additional Information regarding the error can be found in the response body:

{
    "resourceType": "OperationOutcome",
    "id": "1b6335849153345badc0ffeedeadbeef",
    "meta": {
        "lastUpdated": "2023-04-20T08:24:19.9243218+00:00"
    },
    "issue": [
        {
            "severity": "error",
            "code": "invalid",
            "diagnostics": "Value cannot be null.  Parameter name: Patient.gender | Person.gender | Practitioner.gender | RelatedPerson.gender."
        }
    ]
}

[1] https://www.hl7.org/fhir/extensibility.html#primitives

AB#102748

[LTA-Thinking]

This seems to be an issue in our CodeToTokenSearchValueConverter class. When passed a value that doesn't have a code or system it is returning an array with a null element instead of an empty array. Our TypedElementSearchIndexer then throws an exception because it can't work with a null value. The DateToDateTimeSearchValueConverter is returning an empty array, which is why _birthDate is working.

[nielsrustenburg]

We are experiencing the same issue

Fhir-server Version: 3.4.143
Dataprovider: CosmosDB
Fhir Version: STU3