fix(text-field): correctly check _initialHeight

TypeScript 3.7 flags the assignment on line 250 because `style.height` must not be `null`.

The underlying reason though appears to be that the comparison checks against `=== undefined`, where the field only declares `null` as an option. The field is not initialized, so `undefined` is the correct value to compare to and type to use.

Author: mprobst

src/cdk/text-field/autosize.ts

@@ -36,7 +36,7 @@ import {fromEvent, Subject} from 'rxjs';
 export class CdkTextareaAutosize implements AfterViewInit, DoCheck, OnDestroy {
   /** Keep track of the previous textarea value to avoid resizing when the value hasn't changed. */
   private _previousValue?: string;
-  private _initialHeight: string | null;
+  private _initialHeight: string | undefined;
   private readonly _destroyed = new Subject<void>();
 
   private _minRows: number;