Set `mtime` to null instead of false if `getlastmodified` does not exist.
Set `mtime` to null instead of false if `getlastmodified` does not exist.

Signed-off-by: Max Kunzelmann <a47727b1b5981b3d8aae9ac52d1150413c6171dd@posteo.de>

nextcloud/server,nextcloud/server,nextcloud/server,nextcloud/server