Before this patch, the parser mixed mixed "Null" and "Empty" as names
for null placeholder instances in ParserBase. Confusingly, those meant
different things in case of Statements.

This patch uses "Null" anywhere the meaning is "== nullptr" in the Parser.
It also makes use of std::nullptr_t and templatized IsNull() methods to
reduce the amount of boilerplate needed in both the Parser and PreParser.

Change-Id: I1451ba56d1a56466beb7e0c91dcf8e2bb7084413
Reviewed-on: https://chromium-review.googlesource.com/618167
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47406}