fix: preserve Bedrock tool call arguments by removing truthy default

func_info.get('arguments', '{}') returns '{}' (truthy) when no
'function' wrapper exists (Bedrock format), causing the or-fallback
to tool_call.get('input', {}) to never execute. The actual Bedrock
arguments are silently discarded.

Remove the default so get('arguments') returns None (falsy) when
there's no function wrapper, allowing the or-chain to correctly
fall through to Bedrock's 'input' field.

Fixes #5275