Add [TreatReturnedNullStringAs] support for ScalarValueString

Copy/Paste&Find/Replace of r176370 - same thing for ByteString.

Not strictly needed by any existing IDLs, but trivial code change,
and no code size difference until used.

R=horo@chromium.org,haraken@chromium.org
BUG=379009

Review URL: https://codereview.chromium.org/344693002

git-svn-id: svn://svn.chromium.org/blink/trunk@176477 bbb929c8-8fbe-4397-9dbb-9b2b20218538