json-schema-to-grammar : convert empty anyOf / oneOf / type unions instead of emitting an empty rule

[professorpalmer]

Overview

A JSON schema with an empty union ("anyOf": [], "oneOf": [] or "type": []) makes json-schema-to-grammar emit a
rule with no alternatives. For a root schema that is root ::= followed by the space rule; the grammar parser rejects
it and the request fails with Failed to initialize samplers: failed to parse grammar. Reproduces on prism with any
model:

curl -s localhost:8080/v1/completions -d '{"prompt":"x","max_tokens":4,"json_schema":{"anyOf":[]}}'
# {"error":{"code":400,"message":"Failed to initialize samplers: failed to parse grammar",...}}

Agent frameworks emit these unions from generated schemas (reported against a Bonsai 2 27B serve by Milor123 running
Hermes Agent: professorpalmer/bonsai-ada-surgery#3), and the failure ends the agent's turn. The same happens when the
empty union sits inside a tool parameter's schema.

This PR drops an empty union keyword and converts the rest of the schema, in the C++ converter and in
examples/json_schema_to_grammar.py (kept in sync, as test-json-schema-to-grammar runs both). An empty anyOf on
its own therefore converts like {}; {"type": [], "properties": {...}} converts like the object without type.

Additional information

• Upstream ggml-org/llama.cpp handles this since its schema rewrite (common : implement common_schema internal representation for JSON schemas ggml-org/llama.cpp#28736, merged 2026-09-12),
  which prism has not synced yet: there the empty union is rejected up front with a clear "must not be empty" schema
  error. So the choice for this fork is between this conversion (the request succeeds) and backporting upstream's
  rejection (the request still fails, with a readable message). I went with conversion because the reported failure is
  an agent loop that cannot recover from a 400, and the keyword constrains nothing; happy to switch to a minimal
  "reject with a clear error" version instead if you prefer to match upstream until the next sync.
• Tests: four cases added to tests/test-json-schema-to-grammar.cpp (empty anyOf, empty oneOf, empty type with
  properties, empty unions inside properties). The full test passes for C++, Python and the expectation-validity pass;
  without the C++ change the new empty anyOf case fails. Run on our fork, whose three touched files are
  byte-identical to prism at 6bfcd79.
• Served check on an RTX 4070 (Bonsai 2 27B PTQ1_0): every empty-union shape above returns 200, and greedy output is
  unchanged on unrelated prompts.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES. An AI coding assistant reproduced the bug, located the empty rule in the converter, drafted
  the change and the test cases and ran the tests; I reviewed the change and the test output and can explain both.