Doesn't emit validation code for optional fields

[jpeirson]

Take for example this RDL fragment:

type SimpleName String (pattern="[a-zA-Z_][a-zA-Z_0-9]*");
type StringTest Struct {
     SimpleName name;
     SimpleName opt (optional);
}

Running rdl generate go-model for this RDL produces the validation func as:

func (self *StringTest) Validate() error {
	if self.Name == "" {
		return fmt.Errorf("StringTest.name is missing but is a required field")
	} else {
		val := rdl.Validate(TestSchema(), "SimpleName", self.Name)
		if !val.Valid {
			return fmt.Errorf("StringTest.name does not contain a valid SimpleName (%v)", val.Error)
		}
	}
	return nil
}

Although opt is an optional field, it is contrained by a regex pattern that should be validated inside Validate(). I'd expect the validation func to instead be:

func (self *StringTest) Validate() error {
	if self.Name == "" {
		return fmt.Errorf("StringTest.name is missing but is a required field")
	} else {
		val := rdl.Validate(TestSchema(), "SimpleName", self.Name)
		if !val.Valid {
			return fmt.Errorf("StringTest.name does not contain a valid SimpleName (%v)", val.Error)
		}
	}

	// --- additional expected code ---
	if self.Opt != "" {
		val := rdl.Validate(TestSchema(), "SimpleName", self.Opt)
		if !val.Valid {
			return fmt.Errorf("StringTest.opt does not contain a valid SimpleName (%v)", val.Error)
		}
	}
	// ---------------------
	
	return nil
}

[jpeirson]

Created PR #66 that I think fixes this issue without regressions.