Many String FhirPath functions (e.g. startsWith) fail with extension only value

[brianpos]

Describe the bug
When an "extension only" value is provided to the startsWith('xxx') function, a null reference exception is thrown.
This is due to the code going direct to the system string functions without handling any of the null stuff.

firely-net-sdk/src/Hl7.Fhir.Base/FhirPath/Expressions/SymbolTableInit.cs

Line 159 in 5dbd1e5

t.Add("startsWith", (string f, string fragment) => f.StartsWith(fragment), doNullProp: true);

To Reproduce
Steps to reproduce the behavior:
Run the below expression on this test instance

{
    "resourceType": "Patient",
    "id": "example",

    "managingOrganization": {
        "_reference": { "id": "element id" }
    }
}

Can test this in the fhirpath lab too.

managingOrganization.trace('v').reference.startsWith('#')

🧪 Test with FHIRPath-Lab

Returns the error:
Invalid expression: Invocation of function 'startsWith' failed: Object reference not set to an instance of an object.

Expected behavior
This not throw an error and return an empty set as there is no primitive string.
(check the spec, there are some other issues around this at the moment also...)
https://chat.fhir.org/#narrow/channel/179266-fhirpath/topic/result.20of.20toString.20with.20extension.20only.20value

Version used:

• FHIR Version: R4
• Version: v5.10.0