According to the Torque definition, the type of the 'source' field is
'String|Undefined'. The Factory only allowed to pass a string though,
which forced us to set an empty string for wasm scripts.

This CL changes the factory to also allow undefined values, which fits
slightly better for wasm.
The inspector needed a minor change to handle undefined source like an
empty string.

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

Bug: chromium:1125986
Change-Id: Iac0a5ee3767ce121aba8a6a2afe37195e77122fa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2584243Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71913}