firstof tag doesn't require an argument when used with as var

[Lily]

The {% firstof %} template tag raises a TemplateSyntaxError("'firstof' statement requires at least one argument") when called with no arguments.

However, when called with exactly two arguments that match the variable capturing syntax (e.g. {% firstof as foo %}) no error is raised and the FirstOfNode is returned with an empty vars list (and an asvar of e.g. foo). It would be consistent to raise a TemplateSyntaxError("'firstof' statement requires at least one argument before 'as'") here.

This was discovered when ​re-implementing `firstof` in django-rusty-templates.