bug(mat-select): accessing `selected` field on a MatSelect instance causes exception

[AndrewKushnir]

Reproduction

The following code in a template:

<mat-select
  [matTooltip]="$any(select.selected)?.viewValue"
  #select>
</mat-select>

results in the JS error below:

core.umd.js:6824 ERROR TypeError: Cannot read property 'selected' of undefined
    at MatSelect.get (material-select.umd.js:870)
    at ExampleComponent_Template (VM322 example-component.ts:18)
    at executeTemplate (core.umd.js:10101)
    at refreshView (core.umd.js:9967)
    at refreshComponent (core.umd.js:11140)
    at refreshChildComponents (core.umd.js:9763)
    at refreshView (core.umd.js:10017)
    at renderComponentOrTemplate (core.umd.js:10081)
    at tickRootContext (core.umd.js:11314)
    at detectChangesInRootView (core.umd.js:11339)

Stackblitz repro: https://stackblitz.com/edit/components-issue-8aohka?file=src%2Fapp%2Fexample-component.html

The problem is coming from this code:

components/src/material/select/select.ts

Lines 626 to 628 in 8424209

	get selected(): MatOption | MatOption[] {
	return this.multiple ? this._selectionModel.selected : this._selectionModel.selected[0];
	}

where the this._selectionModel is undefined when we try to access it. There is an empty function which checks for the `` presence:

components/src/material/select/select.ts

Lines 764 to 766 in 8424209

	get empty(): boolean {
	return !this._selectionModel || this._selectionModel.isEmpty();
	}

The empty function is used as a guard in some other internal functions in MatSelect, so it can probably be added to the selected getter too. However the signature of the selected getter (MatOption | MatOption[]) doesn't account for the fact that there might be no selected option at all, so this would likely be a breaking change (from types perspective, since it's a public API).

The issue was originally reported internally (internal reference).

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