This makes sure that all overrides of {StackFrame::unchecked_code}
return a value, even if there is no {Code} object associated with the
frame. This ensures debug functions like {StackTraceFailureMessage}
continue working for all stack traces.

R=neis@chromium.org
BUG=chromium:952761

Change-Id: Ie42b301e4d43ebf67acc80e6c1b7bcb4cdc7c947
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1566515Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60844}