Fix the behavior of undefined commands in BlockImageVerify

In BlockImageVerify some commands are undefined, e.g. "erase", "new",
"zero". And we should not error out if the corresponding function
pointer of these commands is null; otherwise we will fail the
verification.

The old code is:
if (cmd->f != nullptr && cmd->f(params) == -1)
  return false;

In the last_command_file change the logic was wrongly modified to
if (cmd->f == nullptr)
  return false;
...
if (cmd->f(params) == -1)
  return false;

Test: sideload an incremental OTA twice on bullhead
Change-Id: I2561c365badb850da0e416629ccd61f0df7da5d7