Entity Data Query -> use emptu string instead of null for latest values

dao/src/main/java/org/thingsboard/server/dao/sql/query/EntityDataAdapter.java

@@ -65,7 +65,7 @@ public class EntityDataAdapter {
                 String strValue;
                 long ts;
                 if (entityKey.getType().equals(EntityKeyType.ENTITY_FIELD)) {
-                    strValue = value != null ? value.toString() : null;
+                    strValue = value != null ? value.toString() : "";
                     ts = System.currentTimeMillis();
                 } else {
                     strValue = convertValue(value);
@@ -99,7 +99,7 @@ public class EntityDataAdapter {
             }
             return strVal;
         } else {
-            return null;
+            return "";
         }
     }