fix(datetime): add correct null check when value changes

[liamdebeasi]

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?

Issue URL: resolves #25714

This fixes a regression caused by the multiple datetime PR. In particular, this line caused the bug: 81d0f3b#diff-4a407530c60e3cf72bcc11acdd21c4803a94bf47ea81b99e757db1c93d2735b8R344.

When defined, hour is a number. When the value prop changed and 12 AM was selected, hour was 0. This caused the ampm variable to be set to undefined because 0 is falsey. When the AM/PM picker column updated, it received undefined as the new value. Whenever the picker column re-renders, it checks to see what the active item is. In this case, it has no active item for value undefined, so it attempts to get itself back into a valid state by selecting the 0th item in the items array:

ionic-framework/core/src/components/picker-column-internal/picker-column-internal.tsx

Line 196 in 3878bf7

this.setValue(items[0].value);

As a result, this causes the picker column to emit 'am' as the value again (since it is the 0th item in the items array). This causes the buggy code to run, forcing the ampm value back to undefined, causing the picker column to re-render again.

What is the new behavior?

• Updated the relevant code to properly check for null or undefined values instead of falsey values.

Does this introduce a breaking change?

• Yes
• No

Other information