valueFhirPath should not default to "value" in case its empty within the Mapping

[geloro94]

In:
src/main/java/de/numcodex/sq2cql/model/Mapping.java
@JsonCreator
public static Mapping of(@JsonProperty("key") TermCode key,
@JsonProperty("fhirResourceType") String resourceType,
@JsonProperty("valueFhirPath") String valueFhirPath,
@JsonProperty("fixedCriteria") Modifier... fixedCriteria) {
return new Mapping(Objects.requireNonNull(key), Objects.requireNonNull(resourceType), valueFhirPath,
return new Mapping(requireNonNull(key), requireNonNull(resourceType),
valueFhirPath == null ? "value" : valueFhirPath,

We should not default to "value". If no valueFhirPath is given the default is not value. This could prevent us from failing fast. Once the mapping is fixed, this should be refactored.