Kotlin. Support stored nulls for nullable fields (#1212)

JAVA-5134

Author: rozza

bson-kotlin/src/main/kotlin/org/bson/codecs/kotlin/DataClassCodec.kt

@@ -85,6 +85,8 @@ internal data class DataClassCodec<T : Any>(
                 if (logger.isTraceEnabled) {
                     logger.trace("Found property not present in the DataClass: $fieldName")
                 }
+            } else if (propertyModel.param.type.isMarkedNullable && reader.currentBsonType == BsonType.NULL) {
+                reader.readNull()
             } else {
                 try {
                     args[propertyModel.param] = decoderContext.decodeWithChildContext(propertyModel.codec, reader)

bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/DataClassCodecTest.kt

@@ -126,6 +126,9 @@ class DataClassCodecTest {
     fun testDataClassWithNulls() {
         val dataClass = DataClassWithNulls(null, null, null)
         assertRoundTrips(emptyDocument, dataClass)
+
+        val withStoredNulls = BsonDocument.parse("""{"boolean": null, "string": null, "listSimple": null}""")
+        assertDecodesTo(withStoredNulls, dataClass)
     }
 
     @Test