fix unit test

Author: thjaeckle

protocol/src/test/java/org/eclipse/ditto/protocol/adapter/things/ThingMigrateCommandAdapterTest.java

@@ -13,6 +13,8 @@
 package org.eclipse.ditto.protocol.adapter.things;
 
 
+import java.util.Collections;
+
 import org.eclipse.ditto.base.model.json.JsonSchemaVersion;
 import org.eclipse.ditto.json.JsonObject;
 import org.eclipse.ditto.protocol.Adaptable;
@@ -54,7 +56,7 @@ public void migrateThingDefinitionFromAdaptable() {
                 thingId,
                 definitionUrl,
                 migrationPayload,
-                null,
+                Collections.emptyMap(),
                 false,
                 TestConstants.DITTO_HEADERS_V_2
         );
@@ -82,7 +84,7 @@ public void migrateThingDefinitionToAdaptable() {
                 thingId,
                 definitionUrl,
                 migrationPayload,
-                null,
+                Collections.emptyMap(),
                 true,
                 TestConstants.DITTO_HEADERS_V_2
         );