fix: handle None inputSchema in extract_mcp_commands

[Christian-Sidak]

Summary

• Guard against inputSchema: null (or missing key) in extract_mcp_commands

Problem

When a non-conformant MCP server returns null for inputSchema, the call
tool.get("inputSchema", {}) returns None (the key exists, its value is
None). The subsequent schema.get("required", []) then raises:

AttributeError: 'NoneType' object has no attribute 'get'

This crashes mcp2cli --list (and any other code path that calls
extract_mcp_commands) for any server that omits or nullifies inputSchema.

Fix

Change line 848 in src/mcp2cli/__init__.py from:

schema = tool.get("inputSchema", {})

to:

schema = tool.get("inputSchema") or {}

or {} treats both a missing key and an explicit None value as an empty
schema, so tools without parameters are listed without error.

Test plan

• test_none_input_schema -- tool with inputSchema=None produces a CommandDef with no params
• test_missing_input_schema -- tool with no inputSchema key produces a CommandDef with no params
• Existing test_basic and test_underscore_to_kebab continue to pass

Fixes #41

[Christian-Sidak]

Friendly bump -- let me know if anything needs changing.