[Kotlin] Fix key lookup returning null clashing with default value (#7237)

* [Java] Fix key lookup returning null clashing with default value

A field with key attribute must always be written on the message so it
can be looked up by key. There is a edge case where inserting a key
field with same value as default would prevent it to be written on
the message and later cannot be found when searched by key.

* [Kotlin] Fix key lookup returning null clashing with default value

A field with key attribute must always be written on the message so it
can be looked up by key. There is a edge case where inserting a key
field with same value as default would prevent it to be written on
the message and later cannot be found when searched by key.

Co-authored-by: Derek Bailey <derekbailey@google.com>

tests/KotlinTest.kt

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+import DictionaryLookup.*;
 import MyGame.Example.*
 import optional_scalars.*
 import com.google.flatbuffers.ByteBufferUtil
@@ -79,9 +80,29 @@ class KotlinTest {
 
         TestSharedStringPool()
         TestScalarOptional()
+        TestDictionaryLookup()
         println("FlatBuffers test: completed successfully")
     }
 
+    fun TestDictionaryLookup() {
+        val fbb = FlatBufferBuilder(16)
+        val lfIndex = LongFloatEntry.createLongFloatEntry(fbb, 0, 99.0f)
+        val vectorEntriesIdx = LongFloatMap.createEntriesVector(fbb, intArrayOf(lfIndex))
+        val rootIdx = LongFloatMap.createLongFloatMap(fbb, vectorEntriesIdx)
+
+        LongFloatMap.finishLongFloatMapBuffer(fbb, rootIdx)
+        val map = LongFloatMap.getRootAsLongFloatMap(fbb.dataBuffer())
+        assert(map.entriesLength == 1)
+
+        val e = map.entries(0)!!
+        assert(e.key == 0L)
+        assert(e.value == 99.0f)
+
+        val e2 = map.entriesByKey(0)!!
+        assert(e2.key == 0L)
+        assert(e2.value == 99.0f)
+    }
+
     fun TestEnums() {
         assert(Color.name(Color.Red.toInt()) == "Red")
         assert(Color.name(Color.Blue.toInt()) == "Blue")