echo -n "" > /sys/module/<modulename>/parameters/<param>

doesn't really work as it will just create a open() + close() expecting
the file to be truncated. The same issue happens with igt as it will
stop writing when there are 0 chars to write. Special case the empty
string so it always write a '\0' and make sure igt_sysfs_set() accounts
for the extra null char.

Shell example:
	# echo -n "/foo" > /sys/module/firmware_class/parameters/path
	# cat /sys/module/firmware_class/parameters/path
	/foo
	# echo -n "" > /sys/module/firmware_class/parameters/path
	/foo
	# # make sure to actually write a \0:
	echo -ne "\0" > /sys/module/firmware_class/parameters/path
	# cat /sys/module/firmware_class/parameters/path

Same thing happens when testing igt_sysfs_set():
       int dirfd = open("/sys/module/firmware_class/parameters", O_RDONLY);
       igt_sysfs_set(dirfd, "path", "");

Previously it was not really setting the param.

v2:
  - Fix return code from igt_sysfs_vprintf() to differentiate between
    writing 1 or 0 chars (Janusz)
  - Document the behavior of igt_sysfs_set() as being a higher-level
    than igt_sysfs_write().
v3:
  - Partial "back to v1": include the \0 in the write only when writing
    an empty string: there are some files in sysfs like pm_test that
    don't like
v4:
  - Fix return code in igt_sysfs_vprintf() when fmt string is something
    like "%c<more_fmt>", c, ..." so it still returns > 0 (Janusz)

Reviewed-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Link: https://lore.kernel.org/r/20240228223134.3908035-4-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>