Avoid calling memcpy from empty string views
Avoid calling memcpy from empty string views

ubsan is unhappy when passing a nullptr as the source pointer to
memcpy even if the length is 0.

Fixes #4720

jkonecny12/kakoune,mawww/kakoune,jkonecny12/kakoune,mawww/kakoune,jkonecny12/kakoune,mawww/kakoune,mawww/kakoune,jkonecny12/kakoune