verity: Fix the broken 'build_verity_tree -A'.

verity: Fix the broken 'build_verity_tree -A'.

Commit 6eb049322c26431961fb69168a113bfd27016ab3 introduced a bug in
checking if there's any passed-in salt string (via '-A <salt>' or '-a
<salt>'). "salt.data()" should be "salt.empty()", which otherwise may
or may not be nullptr even if salt is empty. This CL fixes the issue.

This CL also removes the duplicate variable 'salt_size', which can be
fully covered by salt.size().

Test: m build_verity_tree;
Test: `build_verity_tree system.img verity.img` gives random root hash
      and salt string across runs.
Test: `build_verity_tree -A <hex string> system.img verity.img` gives
      identical results across runs.
Test: `build_verity_tree -a <string> system.img verity.img` gives
      identical results across runs.
Change-Id: I377e42fc48c0f703dc33d813466f6ef60514bbe5