Fix property constraint violation on ConfigurationKey

[lorenzodonini]

Fixes the immediate issue identified in #175.

The validation for the ConfigurationKey.Value field now is run only when the value is not nil. This will prevent a PropertyConstraintViolation error to be thrown, when a nil value is set for a specific key.

[andig]

Still strange that it did even trigger, isn‘t it?

[lorenzodonini]

I guess it's specific for pointer fields. I found a related issue that seems to confirm the behavior.

By quickly checking the source code it also seems to make sense:

func (v *validate) traverseField(ctx context.Context, parent reflect.Value, current reflect.Value, ns []byte, structNs []byte, cf *cField, ct *cTag) {
	var typ reflect.Type
	var kind reflect.Kind

	current, kind, v.fldIsPointer = v.extractTypeInternal(current, false)

	switch kind {
	case reflect.Ptr, reflect.Interface, reflect.Invalid:

		if ct == nil {
			return
		}

		if ct.typeof == typeOmitEmpty || ct.typeof == typeIsDefault {
			return
		}

		if ct.hasTag {
			if kind == reflect.Invalid {
				v.str1 = string(append(ns, cf.altName...))
				if v.v.hasTagNameFunc {
					v.str2 = string(append(structNs, cf.name...))
				} else {
					v.str2 = v.str1
				}
				v.errs = append(v.errs,
					&fieldError{
						v:              v.v,
						tag:            ct.aliasTag,
						actualTag:      ct.tag,
						ns:             v.str1,
						structNs:       v.str2,
						fieldLen:       uint8(len(cf.altName)),
						structfieldLen: uint8(len(cf.name)),
						param:          ct.param,
						kind:           kind,
					},
				)
				return
			}

			v.str1 = string(append(ns, cf.altName...))
			if v.v.hasTagNameFunc {
				v.str2 = string(append(structNs, cf.name...))
			} else {
				v.str2 = v.str1
			}
			if !ct.runValidationWhenNil {
				v.errs = append(v.errs,
					&fieldError{
						v:              v.v,
						tag:            ct.aliasTag,
						actualTag:      ct.tag,
						ns:             v.str1,
						structNs:       v.str2,
						fieldLen:       uint8(len(cf.altName)),
						structfieldLen: uint8(len(cf.name)),
						value:          current.Interface(),
						param:          ct.param,
						kind:           kind,
						typ:            current.Type(),
					},
				)
				return
			}
		}

There is actually a way to achieve the same result by passing a callValidationEvenIfNull = true flag to https://pkg.go.dev/github.com/go-playground/validator/v10#Validate.RegisterValidation, however this isn't the case for built-in types (such as in this issue). I think just explicitly setting omitempty|required will do.