namespace: fix mode for TemporaryFileSystem=

namespace: fix mode for TemporaryFileSystem=

... when no mount options are passed.

Change the code, to avoid the following failure in the newly added tests:

exec-temporaryfilesystem-rw.service: Executing: /usr/bin/sh -x -c
'[ "$(stat -c %a /var)" == 755 ]'
++ stat -c %a /var
+ '[' 1777 == 755 ']'
Received SIGCHLD from PID 30364 (sh).
Child 30364 (sh) died (code=exited, status=1/FAILURE)

(And I spotted an opportunity to use TAKE_PTR() at the end).