AbstractObjectBoxTest: make flex map null by default (for test performance).

Author: greenrobot-team

tests/objectbox-java-test/src/test/java/io/objectbox/AbstractObjectBoxTest.java

@@ -296,11 +296,11 @@ protected TestEntity createTestEntity(@Nullable String simpleString, int nr) {
         entity.setSimpleShortU((short) (100 + nr));
         entity.setSimpleIntU(nr);
         entity.setSimpleLongU(1000 + nr);
-        Map<String, Object> stringObjectMap = new HashMap<>();
         if (simpleString != null) {
+            Map<String, Object> stringObjectMap = new HashMap<>();
             stringObjectMap.put(simpleString, simpleString);
+            entity.setStringObjectMap(stringObjectMap);
         }
-        entity.setStringObjectMap(stringObjectMap);
         return entity;
     }