Only clear share password model when actually saved

Instead of always clearing the password in the share model, now only do
it when it was actually saved.

Fixes an issue where saving another field would clear the password too
early and prevent it to be saved when the dropdown closed.

Signed-off-by: Vincent Petry <vincent@nextcloud.com>
Signed-off-by: npmbuildbot-nextcloud[bot] <npmbuildbot-nextcloud[bot]@users.noreply.github.com>