fix(query-generator): fix bad property access (#10056)

lib/dialects/abstract/query-generator.js

@@ -599,11 +599,10 @@ class QueryGenerator {
     const fieldsSql = options.fields.map(field => {
       if (typeof field === 'string') {
         return this.quoteIdentifier(field);
-      } else if (field._isSequelizeMethod) {
+      }
+      if (field instanceof Utils.SequelizeMethod) {
         return this.handleSequelizeMethod(field);
-      } else {
-        let result = '';
-
+      }
       if (field.attribute) {
         field.name = field.attribute;
       }
@@ -612,9 +611,7 @@ class QueryGenerator {
         throw new Error(`The following index field has no name: ${field}`);
       }
 
-        result += this.quoteIdentifier(field.name);
-        return result;
-      }
+      return this.quoteIdentifier(field.name);
     });
 
     const fieldsSqlQuotedString = fieldsSql.join(', ');