Update PropertyGroup.getProperties to omit properties with null values

Now that convertJsonValue can return null, this is safer/easier than
updating all the callers to be able to handle null.

Signed-off-by: Lee Surprenant <lmsurpre@us.ibm.com>

fhir-config/src/main/java/com/ibm/fhir/config/PropertyGroup.java

@@ -237,12 +237,16 @@ public Object[] getArrayProperty(String propertyName) throws Exception {
     /**
      * Returns the properties contained in the PropertyGroup in the form of a list of
      * PropertyEntry instances. If no properties exist, then an empty list will be returned.
+     * Properties with a value of null will be omitted from the list.
      * @throws Exception
      */
     public List<PropertyEntry> getProperties() throws Exception {
         List<PropertyEntry> results = new ArrayList<>();
         for (Map.Entry<String, JsonValue> entry : jsonObj.entrySet()) {
-            results.add(new PropertyEntry(entry.getKey(), convertJsonValue(entry.getValue())));
+            Object jsonValue = convertJsonValue(entry.getValue());
+            if (jsonValue != null) {
+                results.add(new PropertyEntry(entry.getKey(), convertJsonValue(entry.getValue())));
+            }
         }
         return results;
     }