Add test that verifies the empty string is treated as invalid.
Add test that verifies the empty string is treated as invalid.

This does not change behavior, but rather verifies existing behavior. The existing behavior is a consequence of the regular expression "[ \r\n\t\S]+" being enforce on string primitives.

https://www.hl7.org/fhir/datatypes.html#string

PiperOrigin-RevId: 358023930

google/fhir,google/fhir,google/fhir,google/fhir,google/fhir,google/fhir