JAVA-2349: Handle null values in DBCollection distinct implementation

Author: jyemin

driver/src/main/com/mongodb/DBCollection.java

@@ -1182,6 +1182,9 @@ public List distinct(final String fieldName, final DBCollectionDistinctOptions o
                 .map(new Function<BsonValue, Object>() {
             @Override
             public Object apply(final BsonValue bsonValue) {
+                if (bsonValue == null) {
+                    return null;
+                }
                 BsonDocument document = new BsonDocument("value", bsonValue);
                 DBObject obj = getDefaultDBObjectCodec().decode(new BsonDocumentReader(document), DecoderContext.builder().build());
                 return obj.get("value");

driver/src/test/functional/com/mongodb/DBCollectionFunctionalSpecification.groovy

@@ -338,18 +338,17 @@ class DBCollectionFunctionalSpecification extends FunctionalSpecification {
     def 'should return distinct values of differing types '() {
         given:
         collection.drop()
-        def documents = [~['id' :'a'], ~['id' : 1],
+        def documents = [~['id' : null], ~['id' :'a'], ~['id' : 1],
                          ~['id' : ~['b': 'c']],  ~['id' : ~['list': [2, 'd', ~['e': 3]]]]]
 
         collection.insert(documents)
-        assert collection.count() == 4;
 
         when:
         List distinctValues = collection.distinct('id')
 
         then:
-        distinctValues.size() == 4
-        that distinctValues, contains('a', 1, ~['b': 'c'], ~['list': [2, 'd', ~['e': 3]]])
+        distinctValues.size() == 5
+        that distinctValues, contains(null, 'a', 1, ~['b': 'c'], ~['list': [2, 'd', ~['e': 3]]])
     }
 
     @SuppressWarnings('UnnecessaryObjectReferences')