This isn't the most elegant fix, but I'd prefer to not rework the logic
right now. What happens is:
- Most parts of the Scanner use nullptr to mean, no literal buffer.
- The bookmarking logic may end up with a state where there's a non-nullptr
  literal buffer, but it's empty. (length 0)
- These are functionally equivalent, so there's no 'real' bug.
- But it makes it hard to reason. This patch hence checks for length-0
  literal buffers, and uses nullptr instead.

R=marja@chromium.org
BUG=chromium:639191 v8:4947

Review-Url: https://codereview.chromium.org/2258073003
Cr-Commit-Position: refs/heads/master@{#38756}