[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/DictionaryLookup/LongFloatMap.kt

@@ -0,0 +1,77 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+package DictionaryLookup
+
+import java.nio.*
+import kotlin.math.sign
+import com.google.flatbuffers.*
+
+@Suppress("unused")
+class LongFloatMap : Table() {
+
+    fun __init(_i: Int, _bb: ByteBuffer)  {
+        __reset(_i, _bb)
+    }
+    fun __assign(_i: Int, _bb: ByteBuffer) : LongFloatMap {
+        __init(_i, _bb)
+        return this
+    }
+    fun entries(j: Int) : DictionaryLookup.LongFloatEntry? = entries(DictionaryLookup.LongFloatEntry(), j)
+    fun entries(obj: DictionaryLookup.LongFloatEntry, j: Int) : DictionaryLookup.LongFloatEntry? {
+        val o = __offset(4)
+        return if (o != 0) {
+            obj.__assign(__indirect(__vector(o) + j * 4), bb)
+        } else {
+            null
+        }
+    }
+    val entriesLength : Int
+        get() {
+            val o = __offset(4); return if (o != 0) __vector_len(o) else 0
+        }
+    fun entriesByKey(key: Long) : DictionaryLookup.LongFloatEntry? {
+        val o = __offset(4)
+        return if (o != 0) {
+            DictionaryLookup.LongFloatEntry.__lookup_by_key(null, __vector(o), key, bb)
+        } else {
+            null
+        }
+    }
+    fun entriesByKey(obj: DictionaryLookup.LongFloatEntry, key: Long) : DictionaryLookup.LongFloatEntry? {
+        val o = __offset(4)
+        return if (o != 0) {
+            DictionaryLookup.LongFloatEntry.__lookup_by_key(obj, __vector(o), key, bb)
+        } else {
+            null
+        }
+    }
+    companion object {
+        fun validateVersion() = Constants.FLATBUFFERS_2_0_0()
+        fun getRootAsLongFloatMap(_bb: ByteBuffer): LongFloatMap = getRootAsLongFloatMap(_bb, LongFloatMap())
+        fun getRootAsLongFloatMap(_bb: ByteBuffer, obj: LongFloatMap): LongFloatMap {
+            _bb.order(ByteOrder.LITTLE_ENDIAN)
+            return (obj.__assign(_bb.getInt(_bb.position()) + _bb.position(), _bb))
+        }
+        fun createLongFloatMap(builder: FlatBufferBuilder, entriesOffset: Int) : Int {
+            builder.startTable(1)
+            addEntries(builder, entriesOffset)
+            return endLongFloatMap(builder)
+        }
+        fun startLongFloatMap(builder: FlatBufferBuilder) = builder.startTable(1)
+        fun addEntries(builder: FlatBufferBuilder, entries: Int) = builder.addOffset(0, entries, 0)
+        fun createEntriesVector(builder: FlatBufferBuilder, data: IntArray) : Int {
+            builder.startVector(4, data.size, 4)
+            for (i in data.size - 1 downTo 0) {
+                builder.addOffset(data[i])
+            }
+            return builder.endVector()
+        }
+        fun startEntriesVector(builder: FlatBufferBuilder, numElems: Int) = builder.startVector(4, numElems, 4)
+        fun endLongFloatMap(builder: FlatBufferBuilder) : Int {
+            val o = builder.endTable()
+            return o
+        }
+        fun finishLongFloatMapBuffer(builder: FlatBufferBuilder, offset: Int) = builder.finish(offset)
+        fun finishSizePrefixedLongFloatMapBuffer(builder: FlatBufferBuilder, offset: Int) = builder.finishSizePrefixed(offset)
+    }
+}