Fhirpath toString function not to spec

[brianpos]

• Returns the text "null" when it encounters a primitive datatype that has no value and just an extension
• Returns a comma seperated list of values when run on a collection (spec says to throw an error)

Tracker to clarify behaviour in the core spec here:
https://jira.hl7.org/browse/FHIR-48737

[brianpos]

Unit tests for the expected result: (will also add a new one to the fhirpath test suite...)

  @Test
  public void testEvaluate_ToStringOnDateValue() {
    Patient input = new Patient();
    var dtv = new DateType("2024");
    input.setBirthDateElement(dtv);
    List<Base> results = fp.evaluate(input, "Patient.birthDate.toString()");
    assertEquals(1, results.size());
    assertEquals("2024", results.get(0).toString());
  }

  @Test
  public void testEvaluate_ToStringOnExtensionOnlyValue() {
    Patient input = new Patient();
    var dtv = new DateType();
    input.setBirthDateElement(dtv);
    List<Base> results = fp.evaluate(input, "Patient.birthDate.toString()");
    assertEquals(0, results.size());
  }

[brianpos]

And the actual implementation to resolve this issue: (assuming that the empty set/exception are the correct results)

  private List<Base> funcToString(ExecutionContext context, List<Base> focus, ExpressionNode exp) {
    List<Base> result = new ArrayList<Base>();
    for (Base item : focus) {
      String value = convertToString(item);
      if (value != null)
        result.add(new StringType(value).noExtensions());
    }

    if (result.size() > 1) {
      throw makeException(exp, I18nConstants.FHIRPATH_NO_COLLECTION, "toString", result.size());
    }
    return result;
  }

And should be cloned in R4, R4B and R5
(happy to create the PR if this is all good)

Have tested locally