Fix chat response job template to use String#empty? instead of #blank? when streaming

[kryzhovnik]

What this does

Use empty? instead of blank? so whitespace-only chunks (e.g., "\n" or " ") aren’t dropped

Why

"\n".blank?  # => true
"\n".empty?  # => false
"".empty?    # => true

For example, LLM output is often whitespace-sensitive (think Markdown): dropping streamed newlines or spaces can change the rendered result, even if HTML would ignore them

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes