Db.add_variable() returns None

### 🔍 Before submitting the issue

- [x] I have searched among the existing issues
- [x] I am using a Python virtual environment

### 🐞 Description of the bug

When we create a variable the add method is not returning the variable object.
But documentation example seems to show that it should be true
https://edb.core.docs.pyansys.com/version/stable/api/_autosummary/ansys.edb.core.database.Database.add_variable.html#ansys.edb.core.database.Database.add_variable

### 📝 Steps to reproduce

NA

### 💻 Which operating system are you using?

Windows

### 📀 Which ANSYS version are you using?

NA

### 🐍 Which Python version are you using?

3.9

### 📦 Installed packages

```shell
NA
```

Start at the Database.add_variable() entry point and compare its current return behavior with the linked API documentation example. Reproduce the reported call if possible and verify what value the method returns; done means the implementation and documentation agree on the return value.

Written by the indexing model from the issue text.