Fix HexEncodeTest.TestZeroLengthNoDelimiter with enable_iterator_debugging=true

Fix HexEncodeTest.TestZeroLengthNoDelimiter with enable_iterator_debugging=true,
use_custom_libcxx=false on Windows.

When passed empty string, hex_encode_with_delimiter would dereference
std::string::end() iterator in expression &*s.begin();

Bug: webrtc:10927
Change-Id: I27ce5fecf1f2a5c49a1b85bb94e1dcc92c4c3697
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150651
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Yves Gerey <yvesg@google.com>
Commit-Queue: Kimmo Kinnunen FI <kkinnunen@nvidia.com>
Cr-Commit-Position: refs/heads/master@{#29009}