Adds nullable annotations to weak properties.
Adds nullable annotations to weak properties.

Reviewers: randallli, #material_components_ios_owners

Reviewed By: randallli, #material_components_ios_owners

Projects: #material_components_ios_owners

Differential Revision: http://codereview.cc/D15

randallli/material-components-ios,brianjmoore/material-components-ios,brianjmoore/material-components-ios,chriscox/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,sigito/material-components-ios,Eccelor/material-components-ios,material-components/material-components-ios,sigito/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,Eccelor/material-components-ios,sigito/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,willlarche/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,Eccelor/material-components-ios,willlarche/material-components-ios,chriscox/material-components-ios,samnm/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,material-components/material-components-ios,samnm/material-components-ios,samnm/material-components-ios,chriscox/material-components-ios,brianjmoore/material-components-ios,material-components/material-components-ios,randallli/material-components-ios,brianjmoore/material-components-ios,willlarche/material-components-ios