FIX: Changes in '0' case for reshape position verification

FIX: Changes in '0' case for reshape position verification

Reading sysfs entry that is '0' long should cause an error.
Reshape position cannot be empty.

Absence of reshape position should be ignored. It is possible
that we are about raid0 reshape continuation and it is before takeover.
This means that according metadata (changed by mdmon) it should be reshaped
but md knows nothing about it at this moment. Reshape continuation
in reshape_array() will change it to raid4 and reshape position appears
in sysfs.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>