psplash: Do not mount PSPLASH_FIFO_DIR if the env variable is empty

psplash: Do not mount PSPLASH_FIFO_DIR if the env variable is empty

The script file psplash.sh tries to mount the PSPLASH_FIFO_DIR variable.
If the variable is empty, the mountpoint command returns a usage text
(busybox mountpoint here, util-linux mountpoint behaves the same):

BusyBox v1.37.0 () multi-call binary.
Usage: mountpoint [-q] { [-dn] DIR | -x DEVICE } :~# BusyBox v1.37.0
() multi-call binary

The return code with this console output is 0 and the mount command in the
if statement is executed.
Then this mount also fails with an empty mountpoint argument.
The source code of psplash respects an empty PSPLASH_FIFO_DIR variable
(see psplash.c) and makes a fallback to "/run". So the psplash.sh script should
also respect the empty var.

Try to mount the PSPLASH_FIFO_DIR only if the variable is not empty.

Signed-off-by: Falk Bauer <falkbauer.git@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>