fix: handle null values in YAML output conversion

Co-authored-by: aider (openai/andrew/openrouter/deepseek/deepseek-chat-v3.1) <aider@aider.chat>

src/modes/generate_config.rs

@@ -161,12 +161,19 @@ fn convert_outputs_to_string_map(
 ) -> std::collections::HashMap<String, String> {
     let mut result = std::collections::HashMap::new();
     for (key, value) in outputs {
-        if let Some(str_value) = value.as_str() {
-            result.insert(key.clone(), str_value.to_string());
-        } else {
-            // Convert non-string values to their YAML string representation
+        match value {
+            serde_yaml::Value::Null => {
+                // For null, we want to output "null" as a string
+                result.insert(key.clone(), "null".to_string());
+            }
+            serde_yaml::Value::String(s) => {
+                result.insert(key.clone(), s.clone());
+            }
+            _ => {
+                // Convert other values to their YAML string representation
                 result.insert(key.clone(), serde_yaml::to_string(value).unwrap_or_default());
             }
         }
+    }
     result
 }