Empty schemas (nil schema in OpenAPI specs) represent "any valid JSON value"
according to JSON Schema specification. Previously these were treated as
errors, but they should be handled as ir.Any (jx.Raw in Go).

This commonly occurs in request/response bodies with content but no schema:
{"content": {"application/json": {}}}

Changes:
- Modified schema_gen.go to return ir.Any(nil) for nil schemas
- Added documentation explaining empty schema semantics
- Aligned with existing array handling (line 431)
- Added regression test TestSchemaGenNilSchema
- Moved response_content_schema.json from negative to positive testdata

Fixes #10

Empty schemas (missing schema field in OpenAPI media type objects) now have
context-aware handling:

**Responses**: Generate jx.Raw (any JSON value)
- Allows clients to handle unknown JSON structures from servers
- Solves issue #10 for default error responses and flexible APIs
- Consistent with JSON Schema spec (empty schema = any JSON)

**Requests**: Return error "empty schema in request body"
- Prevents clients from sending arbitrary data without spec guidance
- More conservative approach - requires explicit schema or ignore flag
- Avoids over-permissive APIs where spec doesn't define structure

Changes:
- Added `request bool` field to `generateSchemaOverride` and `schemaGen`
- Modified `generate2()` to check context before handling nil schemas
- Updated `generateContents()` to pass request flag via override
- Split test into separate Response/Request test cases
- Updated test data to use response-only empty schema test

Fixes #10

Empty schemas (missing schema field in OpenAPI media type objects) now have
context-aware handling:

**Responses**: Generate jx.Raw (any JSON value)
- Allows clients to handle unknown JSON structures from servers
- Solves empty response schema issue for default errors and flexible APIs
- Consistent with JSON Schema spec (empty schema = any JSON)

**Requests**: Return error "empty schema in request body"
- Prevents clients from sending arbitrary data without spec guidance
- More conservative approach - requires explicit schema or ignore flag
- Avoids over-permissive APIs where spec doesn't define structure

Changes:
- Added `request bool` field to `generateSchemaOverride` and `schemaGen`
- Modified `generate2()` to check context before handling nil schemas
- Updated `generateContents()` to pass request flag via override
- Split test into separate Response/Request test cases
- Updated test data to use response-only empty schema test

Follows Postel's Law: be liberal in what you accept, conservative in what you send.

Related to ogen-go#10