strip items on null, if default it undefined, set default on null
[yang] strip items on null, if default it undefined, set default on null

Test Plan: read, build, test with some components

Reviewers: nkalashnikov, aschultz, akuleshov

Reviewed By: akuleshov

Subscribers: aschultz, druss, mkraemer

Differential Revision: https://review.tpip.net/D1734