`ngettext` result can be possibly undefined.

[Piotr Kawula]

When the translation engine provide an invalid plural rule the pluralidx function can return invalid index which will cause the ngettext to return value that does not exist - undefined.
Same result can occur when the newcatalog contains invalid values, for example:

const newcatalog = {
  '%s something': [],
  ...
}

And in ngettext we have:

django.ngettext = function(singular, plural, count) {
 const value = django.catalog[singular];
 if (typeof value === 'undefined') {
    return (count == 1) ? singular : plural;
  } else {
    return value.constructor === Array ? value[django.pluralidx(count)] : value;
  }
};

Which in case of empty array go for value[django.pluralidx(count)] which is undefined.

I think we want the ngettext function to return string always, if it does not find proper value in catalog, should default to the provided values.

It affects the previous versions too.