Add a handling for the null case when flattening over the map of sett…

…ings, for example when initial_recovery = null after copying or recovering an index (#3308)

* Add a handling for the null case when flattening over the map of settings, for example when initial_recovery = null after copying or recovering an index

* add tests for null edge case of the Maps class flatten function

* formatting new MapsTest file

elastic4s-domain/src/main/scala/com/sksamuel/elastic4s/ext/Maps.scala

@@ -12,6 +12,7 @@ object Maps {
     case (key, value: java.util.Map[String, V]) => flatten(value.asScala.toMap[String, V]).map { case (k, v) =>
         s"$key$separator$k" -> v
       }
+    case (key, null)                            => Map(key -> null.asInstanceOf[V])
     case (key, value: V)                        => Map(key -> value)
   }
 

elastic4s-domain/src/test/scala/com/sksamuel/elastic4s/MapsTest.scala

@@ -0,0 +1,58 @@
+package com.sksamuel.elastic4s
+
+import com.sksamuel.elastic4s.ext.Maps
+import org.scalatest
+import org.scalatest.matchers.should.Matchers
+
+class MapsTest extends scalatest.flatspec.AnyFlatSpec with Matchers {
+  "Maps flatten function" should "support null values" in {
+    val jsonMap: Map[String, Any] = Map(
+      "settings" -> Map(
+        "index" -> Map(
+          "routing"                -> Map(
+            "allocation" -> Map(
+              "include"          -> Map(
+                "_tier_preference" -> "data_content"
+              ),
+              "initial_recovery" -> Map(
+                "_id" -> null
+              )
+            )
+          ),
+          "number_of_shards"       -> "1",
+          "routing_partition_size" -> "1",
+          "blocks"                 -> Map(
+            "write" -> "true"
+          ),
+          "provided_name"          -> "test-two",
+          "resize"                 -> Map(
+            "source" -> Map(
+              "name" -> "test-index",
+              "uuid" -> "1234"
+            )
+          ),
+          "creation_date"          -> "1234",
+          "number_of_replicas"     -> "1",
+          "uuid"                   -> "1234",
+          "version"                -> Map(
+            "created" -> "1234"
+          )
+        )
+      )
+    )
+    Maps.flatten(jsonMap) shouldBe Map(
+      "settings.index.resize.source.name"                          -> "test-index",
+      "settings.index.resize.source.uuid"                          -> "1234",
+      "settings.index.provided_name"                               -> "test-two",
+      "settings.index.blocks.write"                                -> "true",
+      "settings.index.routing_partition_size"                      -> "1",
+      "settings.index.routing.allocation.include._tier_preference" -> "data_content",
+      "settings.index.routing.allocation.initial_recovery._id"     -> null,
+      "settings.index.number_of_replicas"                          -> "1",
+      "settings.index.version.created"                             -> "1234",
+      "settings.index.number_of_shards"                            -> "1",
+      "settings.index.creation_date"                               -> "1234",
+      "settings.index.uuid"                                        -> "1234"
+    )
+  }
+}