feat: allow nullish compareWith (ng-select#1857)

There are 2 reasons to allow setting compareWith to null/undefined.

1. Reverting the set compareWith - possible the user of the ng-select might want to remove the previously set compareWith to allow default behaviour in combination with bindValue possibly.
2. Extensibility - we have a wrapper around the ng-select that provides custom styles, adds error messages. Not all use cases use compareWith, but some do. Now it is impossible to redirect the [compareWith] @input without setting the default. That hinders the bindValue default behaviour.

src/ng-select/lib/ng-select.component.ts

@@ -131,7 +131,7 @@ export class NgSelectComponent implements OnDestroy, OnChanges, AfterViewInit, C
  get compareWith() { return this._compareWith; }
 
  set compareWith(fn: CompareWithFn) {
- if (!isFunction(fn)) {
+ if (fn !== undefined && fn !== null && !isFunction(fn)) {
  throw Error('`compareWith` must be a function.');
  }
  this._compareWith = fn;