With lazy compilation, not every exported function has code assiciated
with it. {WasmExportedFunction} provides the function index though,
which can be used to check whether code exists and access that code.

R=mstarzinger@chromium.org

Bug: v8:7758
Change-Id: Id80285fec46bf5be4af49875734aa0fe28d732c9
Reviewed-on: https://chromium-review.googlesource.com/1090273Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53576}