In class UserResponse email field is mandatory, but it can be empty which leads to model validation failure

[sergey-ganchuk]

Describe the bug
There are some users that having empty email field
When trying to get those users UserResponse object fails to validate because email is mandatory

To Reproduce
Find user with no email set
try to get it:

artifactory.users.get(user.name)

Expected behavior
it shouldn't fail model validation

python -c "import pyartifactory; print(pyartifactory.version)"
2.7.1

 - Python version, get it with:

python --version

Python 3.8.10

**Additional context**
class UserResponse(BaseUserModel):
    """Models a user response."""

    email: **Optional[EmailStr]** EmailStr
    lastLoggedIn: Optional[datetime] = None
    realm: Optional[str] = None
    offlineMode: bool = False

[anancarv]

Hey @sergey-ganchuk,
When looking at the Artifactory API, I can see that the email field is mandatory. I think we should be compliant with the API documentation and keep the UserResponse as it is.