fix(datetime): don't update value on confirm call if no date was selected

[averyjohnston]

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)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

If you have an ion-datetime with no default value (causing its value to init to undefined), and then call confirm on it without first selecting a date, the value will be updated to today. Because we updated the behavior such that no day is visually pre-selected until you pick one, this no longer makes sense.

Screencast: https://user-images.githubusercontent.com/90629384/169901184-eeb228f1-766b-4ce7-a818-f5d087d094ba.mp4

Issue URL: N/A

What is the new behavior?

The value is not updated on confirm call unless a day is selected first.

Does this introduce a breaking change?

• Yes
• No

Other information