hash: Fix return value for hash_file function
hash: Fix return value for hash_file function

Returning NULL causes a crash (as there is no valid conversion to a
string). Simply return an empty string instead. The rest of the code
is already comparing against an empty string.

Change-Id: I916cf8ac96dd514b8fd59b49c2292afc28b0393b
Reviewed-by: Scott Parlane <85029adc3fa41ef14594a57826619713bfe95f25@alliedtelesis.co.nz>
Reviewed-by: Ridge Kennedy <c60c433c4c1ae237d133641099200771ff6f6f30@alliedtelesis.co.nz>
Signed-off-by: Matt Bennett <e3d8681cec70d3da741935b3385821001b249adc@alliedtelesis.co.nz>

ATL-NZ/buildsyspp,alliedtelesis/buildsyspp