Types generator use constants for hardcoded empty strings

[seseloos]

Replace empty string "" within generator/types_generator.go with constants that have proper names to improve the readability and understanding of the code.

See the following lines of generator/types_generator.go as an example.

apikit/generator/types_generator.go

Lines 317 to 321 in 8d8fa95

	} else {
	typ, err := types.FromSimpleSchema("", &parameter.SimpleSchema, parameter.Required, &parameter.CommonValidations)
	gen.validators.AddAll(typ.GetValidators())
	return typ, err
	}

Also see the suggestion of @donutloop in pull request #5 for further information.