Fix Content-Type header update for non-empty data in set_curl_options

[sarperavci]

Previously, data="" or data=b"" was treated as a real request body, causing Content-Type: application/octet-stream to be added. This was inconsistent with data=None, and it led to overwriting valid user-provided headers.

Issue Example

In parameterized request flows:

http_session = AsyncSession(...)
response = await http_session.request(
    method=METHOD,
    url=URL,
    data=POST_DATA,    # may be "" when no body is intended
    headers=HEADERS,
    verify=False,
    impersonate=impersonate,
    timeout=_timeout,
    default_headers=False,
)

If POST_DATA="", the old behavior forced:

Content-Type: application/octet-stream

which overrode user intent and broke GET requests where an empty body is not actually a payload.

Fix

Only apply the default content-type if the string/bytes payload is non-empty:

- if isinstance(data, (str, bytes)):
+ if isinstance(data, (str, bytes)) and data:

This restores expected behavior:

• data=None → no content-type added
• data="" → no content-type added
• data="foobar" → defaults to application/octet-stream (unchanged)