Add ifExists: to subscripts that return optional

[benasher44]

Changes in this pull request

This adds ifExists: to the subscript methods that return an Optional. This will help, if/when we move forward with Collection conformance for TableViewModel and CollectionViewModel because Collection subscript methods have a non-Optional return type.

Checklist

• [x] All tests pass. Demo project builds and runs.
• [x] I added tests, an experiment, or detailed why my change isn't tested.
• [x] I added an entry to the CHANGELOG.md for any breaking changes, enhancements, or bug fixes.
• [x] I have reviewed the contributing guide

[benasher44]

The littering of ifExists: is a tad annoying, but in most cases, this API shouldn't be used publicly.