Fixes#2965.

Additional description

Updates datepicker to be clearable by setting its value tonull.

CONTRIBUTOR checklist (do not remove)

Please complete for every pull request

• First-time contributors should sign the Contributor License Agreement. It's a fancy way of saying that you are giving away your contribution to this project. If you haven't before, wait a few minutes and a bot will comment on this pull request with instructions.
• npm run lint:fix has been run and linting passes.
• Mocha, Jest (Storyshots), andcomponents/component-docs.json CI tests pass (npm test).
• Tests have been added for new props to prevent regressions in the future. Seereadme.
• Review the appropriate Storybook stories. Openhttp://localhost:9001/.
• Review tests are passing in the browser. Openhttp://localhost:8001/.
• Review markup conforms toSLDS by looking atDOM snapshot strings.

REVIEWER checklist (do not remove)

• CircleCI tests pass. This includes linting, Mocha, Jest, Storyshots, andcomponents/component-docs.json tests.
• Tests have been added for new props to prevent regressions in the future. Seereadme.
• Review the appropriate Storybook stories. Openhttp://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Openhttp://localhost:9001/.
• Review tests are passing in the browser. Openhttp://localhost:8001/.
• Review markup conforms toSLDS by looking atDOM snapshot strings.

Required only if there are markup / UX changes

• Add year-first date and commit SHA tolast-slds-markup-review inpackage.json and push.
• Request a review of the deployed Heroku app by the Salesforce UX Accessibility Team.
• Add year-first review date, and commit SHA,last-accessibility-review, topackage.json and push.
• While the contributor's branch is checked out, runnpm run local-update within locally clonedsite repo to confirm the site will function correctly at the next release.