Fix inadvertent pytest fixture session token usage

This commit will correct the two failing tests:

```text
FAILED tests/cloud/test_object_storage.py::TestObjectStorageClientUnit::
test_generate_presigned_url_example[False]

FAILED tests/cloud/test_object_storage.py::TestObjectStorageClientUnit::
test_generate_presigned_post_example[False]
```

The `False` parameter output means that `use_session_token` was `False`
in the corresponding fixtures in conftest.py:

- `object_storage_config_for_presigned_url_example`
- `object_storage_config_for_presigned_post_example`

There may be situations, such as tests in a GitHub Actions workflow, in
which multiple sets of credentials are present. In these situations,
there may be environment variables such as `AWS_SESSION_TOKEN` that are
set, but should not be used in certain `ObjectStorageConfig` instances.
Setting `session_token` to an empty string (`session_token=""`) prevents
`class ObjectStorageConfig` from using the environment variable value.

There is a unit test, `test_config_with_environment_variable_overrides`
in `tests/cloud/test_object_storage.py`, which verifies the correct
behavior. However, the pytest fixtures listed above were still setting
`session_token=None`, which then was making their `ObjectStorageConfig`
instances inadvertently pick up `AWS_SESSION_TOKEN`. The fix is just to
set `session_token=""` when `use_session_token=False` in those fixtures.

tests/conftest.py

@@ -174,7 +174,7 @@ def object_storage_config_for_presigned_url_example(
             quoted_session_token
         )
     else:
-        session_token = None
+        session_token = ""
     try:
         object_storage_config = fastenv.cloud.object_storage.ObjectStorageConfig(
             access_key="AKIAIOSFODNN7EXAMPLE",
@@ -229,7 +229,7 @@ def object_storage_config_for_presigned_post_example(
             quoted_session_token
         )
     else:
-        session_token = None
+        session_token = ""
     try:
         object_storage_config = fastenv.cloud.object_storage.ObjectStorageConfig(
             access_key="AKIAIOSFODNN7EXAMPLE",