fix MSSQL bug on org (#3405)

vendor/github.com/go-xorm/builder/builder_select.go

@@ -45,6 +45,10 @@ func (b *Builder) selectWriteTo(w Writer) error {
 		}
 	}
 
+	if !b.cond.IsValid() {
+		return nil
+	}
+
 	if _, err := fmt.Fprint(w, " WHERE "); err != nil {
 		return err
 	}