fix(form-field): add a type check for userAriaDescribedBy

* 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 +584,9 @@ export class MatFormField implements AfterViewInit, OnDestroy, AfterContentCheck
     if (this._control) {
       let ids: string[] = [];
 
-      if (this._control.userAriaDescribedBy) {
+      if (this._control.userAriaDescribedBy &&
+        typeof this._control.userAriaDescribedBy === 'string') {
+        console.log('doing split', this._control.userAriaDescribedBy);
         ids.push(...this._control.userAriaDescribedBy.split(' '));
       }
 

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

@@ -507,7 +507,8 @@ export class MatFormField extends _MatFormFieldMixinBase
     if (this._control) {
       let ids: string[] = [];
 
-      if (this._control.userAriaDescribedBy) {
+      if (this._control.userAriaDescribedBy &&
+        typeof this._control.userAriaDescribedBy === 'string') {
         ids.push(...this._control.userAriaDescribedBy.split(' '));
       }