IDL: Avoid generating empty ConstantConfiguration arrays

If all constants in an interface are runtime enabled, an empty
(zero-length) C++ array would be generated, which isn't a good idea.

Review URL: https://codereview.chromium.org/499293003

git-svn-id: svn://svn.chromium.org/blink/trunk@180890 bbb929c8-8fbe-4397-9dbb-9b2b20218538