[jOOQ/jOOQ#9196] Only change Field#add(Field) for non-numeric fields

`a.plus(b).plus(c)` will only for numeric fields be rendered as `(a + b
+ c)` and for non-numeric fields as `((a + b) + c)`. This thus again
restores the original behavior for numeric fields.

jOOQ/src/main/java/org/jooq/impl/Expression.java

@@ -133,9 +133,19 @@ final class Expression<T> extends AbstractField<T> {
         this.arguments = Tools.combine(lhs, rhs);
     }
 
+    @Override
+    public final Field<T> add(Field<?> value) {
+        if (operator == ExpressionOperator.ADD && getDataType().isNumeric()) {
+            rhs.add(value);
+            return this;
+        }
+
+        return super.add(value);
+    }
+
     @Override
     public final Field<T> mul(Field<? extends Number> value) {
-        if (operator == ExpressionOperator.MULTIPLY) {
+        if (operator == ExpressionOperator.MULTIPLY && getDataType().isNumeric()) {
             rhs.add(value);
             return this;
         }