sqlite_utils: don't dereference an empty vector.

user_settings: Add unit test intended to exercise the above path.
In practice though these tests don't hit the bug, since
EncryptData("") results in non-empty ciphertext.

bonus: don't leak a sqlite handle in one of the other sync unit tests.

TEST=none
BUG=40217

Review URL: http://codereview.chromium.org/2801017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@50700 0039d316-1c4b-4281-b951-d872f2087c98