fix(material-experimental/mdc-form-field): address feedback

Author: mmalerba

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

@@ -211,9 +211,10 @@ export class MatFormField
   }
   set appearance(value: MatFormFieldAppearance) {
     const oldValue = this._appearance;
-    this._appearance = ['fill', 'outline'].includes(value)
-      ? value
-      : this._defaults?.appearance || DEFAULT_APPEARANCE;
+    this._appearance =
+      value === 'fill' || value === 'outline'
+        ? value
+        : this._defaults?.appearance || DEFAULT_APPEARANCE;
     if (this._appearance === 'outline' && this._appearance !== oldValue) {
       this._refreshOutlineNotchWidth();