fix(form-field): change undefined check to a type check

* Internal test was failing because userAriaDescribedBy showed up as an object so
  calling .split was throwing errors.

Author: wagnermaciel

src/material-experimental/mdc-form-field/form-field.ts

@@ -584,7 +584,7 @@ export class MatFormField implements AfterViewInit, OnDestroy, AfterContentCheck
     if (this._control) {
       let ids: string[] = [];
 
-      if (this._control.userAriaDescribedBy) {
+      if (typeof this._control.userAriaDescribedBy === 'string') {
         ids.push(...this._control.userAriaDescribedBy.split(' '));
       }
 

src/material/form-field/form-field.ts

@@ -507,7 +507,7 @@ export class MatFormField extends _MatFormFieldMixinBase
     if (this._control) {
       let ids: string[] = [];
 
-      if (this._control.userAriaDescribedBy) {
+      if (typeof this._control.userAriaDescribedBy === 'string') {
         ids.push(...this._control.userAriaDescribedBy.split(' '));
       }