bug(form-field): remove hint wrapper when no hint provided

### Is this a regression?

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

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

_No response_

### Description

Even when no hint is provided, the hint-wrapper dom element still shows up.
This is because `mat-form-field-subscript-wrapper` has a margin as specified here:
```CSS
.mat-form-field-subscript-wrapper {
font-size: $subscript-font-size;
margin-top: $subscript-margin-top;
```

### Reproduction

Steps to reproduce:
1. Add an outline material form field without any hint and inspect.
eg:
```HTML

Outline form field


```

### Expected Behavior

When no hint is being provided, the wrapper dom element shouldn't render.

### Actual Behavior

The wrapper renders and takes up space.

### Environment

- Angular: 13.3.5
- CDK/Material: 13.3.5
- Browser(s): Chrome / all
- Operating System (e.g. Windows, macOS, Ubuntu): macOS/ all

首先复现所提供的 outline mat-form-field 示例，并检查其生成的 DOM，尤其是 mat-form-field-subscript-wrapper 元素及其间距。当不带 hint 的表单字段不渲染该 wrapper 且不再有不必要的空白时，issue 即完成。

由索引模型根据 Issue 内容生成。