Fixed unnecessary warnings.

Since we are looping through each named-breakpoint, we don't need to have `@else` to set the warning.

If the loop reaches its end, then a warning shall be emitted.
Fixed `mq-stringify` adding glue to the end.

The function was having trouble evaluating `not $root`, returning 'true' and appending glue on the end of the string.

Swapped it for `$root != true`.