IDL compiler: [ReflectEmpty]

Python side to:
Empty reflected attributes and string literals in extended attributes.
https://codereview.chromium.org/127903002/

Followup to:
IDL compiler: [ReflectMissing], [ReflectInvalid]
https://codereview.chromium.org/152043003/

This completes [Reflect*] extended attributes.
I've got 2 cleanup CLs to do after this, which will simplify the CG.

Remaining: 9 => 6

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/155623004

git-svn-id: svn://svn.chromium.org/blink/trunk@166464 bbb929c8-8fbe-4397-9dbb-9b2b20218538