Empty function names are allowed and are output as such, unnamed
functions or functions with no valid UTF-8 name are output as
"<WASM UNNAMED>", while the CallSite object returns null as the
function name.

R=titzer@chromium.org, yangguo@chromium.org

Review-Url: https://codereview.chromium.org/1970503004
Cr-Commit-Position: refs/heads/master@{#36348}