fix: panic on oneof validation

[megaflo]

The generated validation for oneof schemas produces an error, if used as a property in another object and left empty (e.g. by omitting that property from the json payload). The cause is a nil initialized interface in the implementation, that gets dereferenced.

This PR adds extra checks to make sure the nil interface is never dereferenced.

Ticket

How Has This Been Verified?

A new test has been added to verify the expected behavior (a validation error is returned for missing oneof fields).

Checklist:

• The change works as expected.
• New code can be debugged via logs.
• I have added tests to cover my changes.
• I have locally run the tests and all new and existing tests passed.
• Requires updates to the documentation.
• I have made the required changes to the documents.

[megaflo]

@LucasRoesler I just realized that this way of fixing the issue effectively makes any oneof field required, as validation will fail, if the property is not provided. Is that something we can live with or should I try to avoid validation of the property if it is not listed as required?

[LucasRoesler]

@LucasRoesler I just realized that this way of fixing the issue effectively makes any oneof field required, as validation will fail, if the property is not provided. Is that something we can live with or should I try to avoid validation of the property if it is not listed as required?

is it possible to toggle the error behavior based on the not null property?

[LucasRoesler]

@megaflo actually now that i think about it, i don't think we need to change anything, the required validation should already work as expected, e.g

if you have

    Container:
      type: object
      properties:
        error:
          $ref: "#/components/schemas/Error"

then

func (m Container) Validate() error {
	return validation.Errors{
		"error": validation.Validate(
			m.Error,
		),
	}.Filter()
}

should skip validation is m.Error is empty, this is part of the behavior of the ozzo-validation