fix(virtual-scroll): allow null in items prop

[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)
• 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?

Using the async pipe is considered best practice for angular projects.

When using the async pipe with <ion-virtual-scroll> with strictTemplates set to true in tsconfig.json we get this error Type 'null' is not assignable to type 'any[] | undefined'. during the build phase.

Issue Number: N/A

What is the new behavior?

When setting angularCompilerOptions.strictTemplates=true in tsconfig.json the build fails because there is a type mismatch between async & [items]. This is because the return type of async can be any[] | null whereas [items] only accepts any[] | undefined. Adding null to the allowed types fixes the issue and allows async to be used with virtual scroll.

Does this introduce a breaking change?

• Yes
• No

Other information