test: use valid empty-symbol literal for null-symbol round-trip

[belowzeroff]

Problem

make test fails on a clean checkout at test/rfl/server/types.rfl:50:

test/rfl/server/types.rfl:50: LHS error: error: name  -- src: (.q.send h "0Ns")

The server type-matrix test and the real-q interop script (test/run.sh) both
assert on 0Ns, but Rayfall has no 0Ns literal — the engine parses it as
a name and signals 'name. (0Nl / 0Ni / 0Nf are valid; only 0Ns is
not.) So the null-symbol row breaks the server round-trip suite.

Fix

Write the null symbol as the empty symbol (as 'sym ""), which the engine
accepts and which round-trips over the wire (a -KS atom with a zero-length
name). Same substitution applied to the real-q interop assertion in
test/run.sh (compared against q's `).

Validation

test/rfl/server/round_trip.rfl           ok (7 assertions)
test/rfl/server/types.rfl                ok (25 assertions)
PASSED: 2 file(s), 0 failure(s)

Empty-symbol round-trip confirmed end-to-end against driver --serve; 0Nl /
0Ni / 0Nf continue to pass.

🤖 Generated with Claude Code