fix(menu, split-pane): ssr does not fail on null customElements check

[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 #24714

SSR builds that used menu/split pane were failing because whenDefined was trying to reference a null customElements.

What is the new behavior?

• Adds a null check for customElements
  Note: I'm not 100% if we only need a null check, so I kept the typeof check and checked for null + undefined just to be safe. Some of the repros in 24714 (and related issues) attempted to polyfill parts of these APIs, but I'm not sure if they all polyfilled them in the same way.

Dev build: 6.5.7-dev.11677189602.12905db0

Does this introduce a breaking change?

• Yes
• No

Other information

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.