Select falsy values via ngModel - master

[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.

• When testing NaN use an additional variable in the ts file

public selectNaN: any = NaN;

<igx-simple-combo [data]="data" [(ngModel)]="selectedValue"></igx-simple-combo>
<button igxButton (click)="selectedValue = selectNaN"> NaN </button>

Assigning it directly in the html file shows an error "Property 'NaN' does not exist on type 'ComboSampleComponent'.ngtsc(2339)" and the variable bound to the ngModel is assumed as 'undefined'.

• Test IgxSimpleCombo when bound to the ngModel with required attribute set:
  • if required attribute is set, null, undefined, '' should not be selected in IgxSimpleCombo, otherwise, null, '' should be selected.

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

[teodosiah]

When the simple combo is bound to a data source like ['', false, NaN, null, undefined] selecting the 'false' value through the combo's drop-down is throwing an error: