bug(MatFormFieldControl, MatInput): enable empty value for element attributes

### Is this a regression?

- [x] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

21

### Description

Recent TypeScript 6 flashes out discrepancies in attributes declaration for html/material controls.

For MatInput, as for other controls, TypeScript doesn't allow empty (null or undefined) values.

The `required`, `placeholder`, etc. forces strict datatype while others such as `autocomplete` are optional.

Previous Angluar version 21 handled these cases correct, for example, placeholder in MatFormField.

In my view, all MatFormFieldControl attributes should be optional, to reflect HTML tag design by definition.

It's common to forward optional attributes between components that holds `undefined` value; forcing an issue when refering to HTML tag such as `input` with Angular decoration.

### Reproduction

StackBlitz link: https://stackblitz.com/edit/components-issue-starter-ffzyyuuu?file=src%2Fmain.ts
Steps to reproduce:
See errors for empty placehoder and missing maxlength.

### Expected Behavior

Attributes such as `placeholder` should accept null and undefined.

This should be common among all MatFormFieldControl implementations.

### Actual Behavior

Common attributes doesn't allow empty values.

### Environment

Angular CLI : 22.0.0
Angular : 22.0.0
Node.js : 24.16.0
Package Manager : npm 11.16.0
Operating System : win32 x64

StackBlitz の再現から始め、MatInput の TypeScript 宣言と、issue で名前が挙げられている MatFormFieldControl の実装を調べます。空の placeholder、maxlength、required、および関連する属性の動作を確認し、その後、null と undefined が一貫して受け入れられるようにカバレッジを追加または更新します。

索引モデルが issue の本文から書いたものです。