TurboFan lowering (see [0]) of ConsString creation cannot ensure that
the first part of the cons string is non-empty without introducing a phi
and negatively impacting performance.

This modifies ConsStringIterator to allow empty first parts of
ConsStrings.

BUG=v8:5440

Review-Url: https://codereview.chromium.org/2377983002
Cr-Commit-Position: refs/heads/master@{#39817}