Twenty Seventeen: Ensure functions in `customize-controls.js` don't count on...

Twenty Seventeen: Ensure functions in `customize-controls.js` don't count on Customizer sections always being present

Before, the code assumed the `theme_options` section is always present, but it can be removed by plugins. This way, if it is, no JavaScript errors occur.

Props westonruter.

Fixes #39335.

Built from https://develop.svn.wordpress.org/trunk@39623


git-svn-id: https://core.svn.wordpress.org/trunk@39563 1a063a9b-81f0-0310-95a4-ce76da25c4cd