[IgxSimpleCombo] Select falsy values - 14.1.x

[RivaIvanova]

Closes #12048

NOTE FOR TESTING : Should be able to select all falsy values except 'undefined', i.e., false, '', 0, NaN, null. For complex data, if there is an item with valueKey=undefined it should be displayed in the dropdown, but cannot be selected.

Additional information (check all that apply):

• [x] Bug fix
• [ ] New functionality
• [ ] Documentation
• [ ] Demos
• [ ] CI/CD

Checklist:

• [x] All relevant tags have been applied to this PR
• [x] 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