test/py: spi: Set the expected error message

If erase/write/read size is 0 then it throws the mentioned error message
when debug message ie enabled as per 899fb5aa ("cmd: sf/nand: Print
and return failure when 0 length is passed"), setting it to None as
debug message is not enabled by default for testing.

Signed-off-by: Love Kumar <love.kumar@amd.com>