For streaming compilation, scripts don't have a source string attached
until finalization, but the Script and SharedFunctionInfo objects are
already on the heap and may be picked up by heap walks.

This happens e.g. in CollectSourcePositionsForAllBytecodeArrays, where
we then try to reparse and recompile the SFI. This is invalid, since
the source string is not yet set.

Avoid this by checking for the empty source string (and leaving a TODO
for a nicer future solution).

Bug: v8:12051
Change-Id: Ib4f40cd218151120e5aff8558dd5df5c8834412e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3071403
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76104}