fix(intel): null pointer handling for resp_len

Previous changes from commit #6a659448 updates resp_len from an integer
type to unsigned integer pointer type. This patch adds proper handling
in case resp_len is a null pointer. Resp_len with value 0 are also
changed to NULL to match the type change.

Signed-off-by: Abdul Halim, Muhammad Hadi Asyrafi <muhammad.hadi.asyrafi.abdul.halim@intel.com>
Change-Id: I75b3e3bfbb188d8e7b329ba3b948c23e31dec490