Fix clearing the label of a share

When the label is being edited its value is stored in "newLabel", and
the original label is shown only if "newLabel" is not set. However, as
it was checked against any falsy value instead of only undefined when
the label was cleared and thus it has an empty value the original label
was shown instead.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>
Signed-off-by: npmbuildbot-nextcloud[bot] <npmbuildbot-nextcloud[bot]@users.noreply.github.com>