fix(core/date-picker): handle empty from date

[philippsteinberg]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (yarn build) was run locally and any changes were pushed
• Unit tests (yarn test) were run locally and passed
• Visual Regression Tests (yarn visual-regression) were run locally and passed
• Linting (npm lint) was run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bug fix
• Feature
• Refactoring (no functional changes, no API changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

If you deselect the the date on the datepicker and press done a js issue is thrown because there is no null check on the toFormat function

GitHub Issue Number: N/A

What is the new behavior?

Null check before the toFormat call

Does this introduce a breaking change?

• Yes
• No

Testing

Other information

[nuke-ellington]

While this can be a fine temporary fix IMO we need to find anther solution like disabling the Done button while no date is being selected e.g. We can adress this in #324.

[danielleroux]

lgtm