autocomplete onModelChange should be be triggered onInput if forceSelection is true

[huineng]

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Current behavior

onModelChange is being emitted after each keystroke (input).. when forceselection is enabled this should not be the case

Expected behavior

in a reactive form we listen to form validations, when an autocomplete value is required, then we want only that control to be valid once the value is selected.

So the onmodelchange should be removed from the oninput if the forceselection is true
https://github.com/primefaces/primeng/blob/master/src/app/components/autocomplete/autocomplete.ts#L303

let value = (<HTMLInputElement> event.target).value;
        if(!this.multiple && ! this.forceSelection) {
            this.onModelChange(value);
        }

[ghost]

The problem with this is that if on the FormControl we have a required Validator if we write a value in autocomplete then manuall delete it (with backspace) the valueChanges is not fired on the form