Honor output buffers for zero-length stream reads

[OskarEichler]

Rackup::Stream::Reader#read(0, buffer) should follow the documented IO-like contract: clear and return the caller's buffer without touching input. The current early return instead leaves the buffer stale and returns a different UTF-8 string. Without a buffer it also returns a frozen UTF-8 literal rather than a mutable binary string consistent with other reads.

Return the cleared caller buffer when provided, otherwise a new ASCII-8BIT string.

This ports the corresponding source-origin corrections from socketry/protocol-http commits ee2318e and 479d0bf, whose Stream implementation Rackup shares.

Verification:

• deterministic buffer/encoding model fails on 2.3.1/current and passes this branch
• full suite: 76 runs / 194 assertions / 0 failures
• Ruby syntax, warning-mode load, package build and diff checks pass on Ruby 4.0.6
• combined candidate also passes 74 runs / 182 assertions on Ruby 3.2.11

No breaking API change; this restores the documented contract.