MRP-1203 mdt: Fix the assert in mdt_lock_pdo_init

Fix the assert in mdt_lock_pdo_init to also take into consideration
the case wherein the variable name is not null but rather an empty
string

Signed-off-by: Cheng Shao <cheng_shao@xyratex.com>
Change-Id: Icc8a732318743a065a1be25c4acffdd872b6d853
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/614
Tested-by: Jenkins
Reviewed-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-by: Rahul Deshmukh <Rahul_Deshmukh@xyratex.com>
Tested-by: Elena Gryaznova <Elena_Gryaznova@xyratex.com>
Reviewed-by: Denis Kondratenko <Denis_Kondratenko@xyratex.com>