Fix NPE in enumeration switch statements (#1846)

* Fix weird double if

* Add test for null doc status and preserving extension

* Add fix for null enum DocumentReference30_40

* Use java null instead of Enumeration.NULL for Enumeration src.setValue

* Apply null pointer check to all switch(Enumeration) statements

org.hl7.fhir.convertors/pom.xml

@@ -86,6 +86,7 @@
             <artifactId>xpp3</artifactId>
             <optional>true</optional>
         </dependency>
+
         <!-- HTTP Client -->
         <dependency>
           <groupId>org.apache.httpcomponents</groupId>
@@ -120,6 +121,12 @@
             <scope>test</scope>
         </dependency>
 
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <scope>test</scope>
+        </dependency>
+
         <dependency>
             <groupId>org.xerial</groupId>
             <artifactId>sqlite-jdbc</artifactId>