Optimize the MYSQL

Src/Asp.Net/SqlSugar/Abstract/ExecuteNavProvider/InsertNavProviderManyToMany.cs

@@ -57,6 +57,10 @@ namespace SqlSugar
                                 {
                                     keyValuePairs.Add(pair.DbColumnName, DateTime.Now);
                                 }
+                                else if (pair.UnderType == UtilConstants.StringType)
+                                {
+                                    keyValuePairs.Add(pair.DbColumnName, UtilConstants.Space);
+                                }
                                 else
                                 {
                                     keyValuePairs.Add(pair.DbColumnName, UtilMethods.GetDefaultValue(pair.UnderType));

Src/Asp.Net/SqlSugar/Abstract/ExecuteNavProvider/UpdateNavManyToMany.cs

@@ -67,6 +67,10 @@ namespace SqlSugar
                             {
                                 keyValuePairs.Add(pair.DbColumnName, DateTime.Now);
                             }
+                            else if (pair.UnderType == UtilConstants.StringType)
+                            {
+                                keyValuePairs.Add(pair.DbColumnName, UtilConstants.Space);
+                            }
                             else
                             {
                                 keyValuePairs.Add(pair.DbColumnName, UtilMethods.GetDefaultValue(pair.UnderType));