Fix empty Select Model button when no model is selected

[heyshreenanda]

Problem

When an AI provider is selected without choosing a specific model,
the Select Model button renders empty text.

Cause

The button label was bound to an empty string value, which bypassed
the null-coalescing fallback.

Fix

Added an explicit empty-string check and fallback to "Select Model".

Testing

• Selected provider without selecting a model → button shows "Select Model"
• Selected a model → button shows model name