Add support for falsy display values to simple combo

[jackofdiamond5]

Closes #10952

When using complex data, values like null, undefined and '' are now supported as display values and will be treated like any other value. The only exceptions are false and NaN which are still not supported.

Additionally, while the display key can be a falsy value, the value key (id) cannot, since it is used by the selection service to determine what item should be added to the selection. If a falsy value is passed in as a value key, then the selection service and respectively the combo will throw an error Invalid value for item id!.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them