This fixes a corner-case in the {NativeModuleSerializer} with modules
that do not contain any functions in the code table.

R=ahaas@chromium.org
TEST=mjsunit/regress/wasm/regress-801850
BUG=chromium:801850

Change-Id: I30cc3a26f30d8653fba2d7b99715830d12300ac2
Reviewed-on: https://chromium-review.googlesource.com/866773Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50581}