time: an empty source_timezone surfaces zoneinfo's raw error #5003

Problem

get_current_time with an empty timezone returns the server's own "Missing required argument" error, but convert_time with an empty source_timezone passes zoneinfo's raw exception text to the client.

Where it is pinned

src/time/tests/test_protocol.py: test_convert_time_argument_errors (the empty source_timezone case), added in #4972 (Wave 1 of #4857). Each of these tests carries a KNOWN BUG marker: it asserts the current, wrong behavior, so the fix has to change it.

Expected

Both tools report an empty timezone argument with the same clear error.

Done when

• The behavior above is fixed, in the legacy-era (2025-11-25) server on SDK 1.x.
• The pinning tests assert the correct behavior, and their KNOWN BUG markers are gone.
• The per-file 90% coverage gate still passes.