Return empty HTTP ranges when the start is omitted

[dlowzzxx]

Summary

cat_file(url, end=0) means an empty slice, but both HTTP implementations currently send Range: bytes=0--1 when start is omitted. The local test server disconnects instead of returning empty bytes.

Return b"" before opening a session in both the async and synchronous implementations. Existing equal-range handling stays unchanged. The base-file fix in #2200 does not cover these HTTP overrides.

Tests

• Both new omitted-start regressions fail before the fix.
• Eight sync/async range cases pass and assert that no GET is sent.
• Full HTTP modules: 170 passed, 1 skipped.
• Core/spec modules: 385 passed, 129 skipped, 1 xfailed.
• Ruff 0.14.3 lint/format and whitespace checks pass.

The Windows runs emit existing HTTP-server shutdown warnings and an asyncio deprecation warning. The full optional-backend matrix was not run locally.

Prepared with OpenAI Codex assistance, disclosed in the commit trailer.