[6.15.z] Fix HTTP proxy teardown

[Satellite-QE]

Cherrypick of PR: #15229

Problem Statement

In #14513 I overlooked that http_proxy can be even None which does not implement the delete() method so the teardown fails with

pytest_fixtures/component/http_proxy.py:32: in setup_http_proxy
    http_proxy.delete()
E   AttributeError: 'NoneType' object has no attribute 'delete'

Solution

Delete only existing objects.

PRT test Cases example

trigger: test-robottelo
pytest: tests/foreman/api/test_http_proxy.py -k test_positive_install_content_with_http_proxy

[Satellite-QE]

trigger: test-robottelo
pytest: tests/foreman/api/test_http_proxy.py -k test_positive_install_content_with_http_proxy

[vsedmik]

trigger: test-robottelo
pytest: tests/foreman/api/test_http_proxy.py -k test_positive_auto_attach_with_http_proxy

[Satellite-QE]

PRT Result

Build Number: 7211
Build Status: SUCCESS
PRT Comment: pytest tests/foreman/api/test_http_proxy.py -k test_positive_auto_attach_with_http_proxy --external-logging
Test Result : ========== 3 passed, 4 deselected, 207 warnings in 1731.92s (0:28:51) ==========