fix(material-experimental/mdc-form-field): ensure appearance is valid

Author: mmalerba

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

@@ -211,7 +211,9 @@ export class MatFormField
   }
   set appearance(value: MatFormFieldAppearance) {
     const oldValue = this._appearance;
-    this._appearance = value || this._defaults?.appearance || DEFAULT_APPEARANCE;
+    this._appearance = ['fill', 'outline'].includes(value)
+      ? value
+      : this._defaults?.appearance || DEFAULT_APPEARANCE;
     if (this._appearance === 'outline' && this._appearance !== oldValue) {
       this._refreshOutlineNotchWidth();