use str.empty() to check for empty string instead of !str[0]

!str[0] is technically speaking reaching past the end of the 0-byte empty string,
though there should normally be an extra 0 byte added to facilitate .c_str()

Bug: 206703697
Test: TreeHugger, atest netd_unit_test
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I88ddae1b00ff020bd833d057468f729fb530216d
1 file changed