fix(modal, popover): warn if trigger element was not found

[liamdebeasi]

Note: I will put up a separate patch for v7 since how triggers are consumed has changed slightly.

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?

There is some developer confusion about how the trigger prop works. See #26648

What is the new behavior?

• Both modal and popover have been updated to a) return early in configureTriggerInteraction if no trigger prop is set and b) warn if a trigger prop is set but the element was not found in the DOM.

Does this introduce a breaking change?

• Yes
• No

Other information

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.