Handle case where the tool-call command is nil

[genegoykhman]

Fixes #297

Checklist

• I've read the README's Contributing section.
• [n/a] I've filed a feature request/discussion for a new feature.
• My code follows the project style.
• [n/a] I've added tests where applicable.
• [n/a] I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.
• I've reviewed all code in PR myself and will vouch for its quality.

Description

When using agent-shell with Anthropic's Claude Code I was getting this error immediately upon issuing my first prompt:

Debugger entered--Lisp error: (error "Unexpected tool-call command type: symbol")
  error("Unexpected tool-call command type: %S" symbol)

It looks like agent-shell--tool-call-command-to-string emits this error when the passed command parameter is nil, which it evidently can be sometimes. This PR simply adds a graceful fallback when command is nil.

[xenodium]

Thank you