fix: make email in UserResponse optional

[ohjinsoo]

Description

I'm trying to get a User from an enterprise Artifactory instance, but I'm getting the following error:

>>> art.users.get("username")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/pyartifactory/objects/user.py", line 48, in get
    return UserResponse(**response.json())
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/pydantic/main.py", line 193, in __init__
    self.__pydantic_validator__.validate_python(data, self_instance=self)
pydantic_core._pydantic_core.ValidationError: 1 validation error for UserResponse
email
  Field required [type=missing, input_value={'name': 'username', 'admin'..., 'shouldInvite': False}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.8/v/missing

and when I query that user manually, I don't see an email field in the response from Artifactory. It's most likely because we get users using Okta, so email never gets set.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

How has it been tested ?

I cloned my fork, ran the poetry install setup in CONTRIBUTING.md, and tried getting the same user. I was able to get the user through this method.

Checklist:

• [x] My PR is ready for prime time! Otherwise use the "Draft PR" feature
• [ ] All commits have a correct title
• [ ] Readme has been updated
• [x] Quality tests are green (see Codacy)
• [ ] Automated tests are green (see pipeline)