LU-4762 lctl: set_param should return error when value is null

LU-4762 lctl: set_param should return error when value is null

"lctl set_param" should return error when the parameter value is null.
For example: "lctl set_param debug_mb" or "lctl set_param fail_loc="

Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: I5572c7a16803f1c4ba4449056b4dd2c476106bde
Reviewed-on: http://review.whamcloud.com/9638
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>