Fall back on user.email if sendemail.from not set

If we don't find a sendemail.from, then fall back to using user.email
and hope for the best.

Reported-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://msgid.link/eb5d38af-ed64-38fd-2515-e489120cbb0b@linaro.org
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>