Avoid masking/unmasking the empty list of mount units (!116)

After the previous commit "Stop masking the root file system mount
unit", GParted now reports this error to the terminal on Debian 10 and
11:
    # gparted
    To few arguments.
    GParted 1.5.0-git
    configuration --enable-online-resize
    libparted 3.2

Debian installations, at least on PC hardware and using BIOS booting so
they don't have a /boot/efi file system, only have a single file system,
/ (root).  That is now excluded from masking so gparted shell wrapper
runs systemctl without any mount units to mask.  Hence the error.
    # systemctl --runtime mask --quiet --
    Too few arguments.
    # echo $?
    1

Fix this by only masking and unmasking units when the list of mounts
is non-empty.

Closes !116 - Systemd mount masking and udev rule location updates

gparted.in

@@ -148,6 +148,7 @@ done
 #
 #  Use systemctl to prevent automount by masking currently unmasked mount points
 #
+MOUNTLIST=''
 if test "x$HAVE_SYSTEMCTL" = "xyes"; then
 	MOUNTLIST=`systemctl show --all --property=Where,What,Id,LoadState '*.mount' | \
 	awk '
@@ -172,8 +173,10 @@ if test "x$HAVE_SYSTEMCTL" = "xyes"; then
 	/^$/          { process_unit() }
 	END           { process_unit() }
 	'`
+	if test "x$MOUNTLIST" != "x"; then
 		systemctl --runtime mask --quiet -- $MOUNTLIST
 	fi
+fi
 
 #
 # Create temporary blank overrides for all udev rules which automatically
@@ -234,7 +237,7 @@ done
 #
 #  Use systemctl to unmask those mount points masked above
 #
-if test "x$HAVE_SYSTEMCTL" = "xyes"; then
+if test "x$HAVE_SYSTEMCTL" = "xyes" && test "x$MOUNTLIST" != "x"; then
 	systemctl --runtime unmask --quiet -- $MOUNTLIST 2> /dev/null || \
 	{
 		cd /run/systemd/system  &&