Simplify Editor::Command::value().

Move valueNull()-specific code from Editor::Command::value() to valueNull(),
and rename valueNull() to valueStateOrNull(), valueNo() to valueEmpty().
This CL has no behavior changes.

Review-Url: https://codereview.chromium.org/2771753005
Cr-Commit-Position: refs/heads/master@{#459377}
1 file changed