fix(mat-form-field): add a check to avoid calling .split when it does not exist

Author: wagnermaciel

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 &&
+        this._control.userAriaDescribedBy.split) {
         ids.push(...this._control.userAriaDescribedBy.split(' '));
       }