fix: prevent nested validations from throwing

[velvet-lynx]

Description

class PostDto {
  @IsString({
    message: i18nValidationMessage<I18nTranslations>('validation.nested'),
  })
  title: string;

  @IsString({
    message: i18nValidationMessage<I18nTranslations>('validation.nested'),
  })
  body: string;
}

export class PostsDto {
  @ValidateNested({ each: true })
  @Type(() => PostDto)
  posts: PostDto[];
}

When using the decorator @ValidateNested({each: true]) from class-validator like in the above example, the I18nService.validate(...) method will throw the following error TypeError: Cannot convert undefined or null to object.

This is because in the absence of validation errors in the parent, the constraints property of the ValidationError will be undefined. Thus, the call to Object.keys(error.constraints) in the formatI18nErrors util function will fail. Since Object.keys only works with objects.

Linked Issues

Fixes #633

Additional context

[coveralls]

coverage: 90.837% (-0.07%) from 90.909%
when pulling 7ad950d on velvet-lynx:fix/validate-nested-each
into c794613 on toonvanstrijp:main.