if we do not want to return a value for submitting, we have to return
'null', every other value will get returned by getSubmitValue,
including 'undefined' and '' (empty string)
this fixes an issue when the user did not enter any bwlimit on
restore (the user would get an api error that bwlimit expects an
integer and not '')
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by: Stefan Reiter <s.reiter@proxmox.com>
