Don't include prompt titles / "Default Prompt:" in slash command output (#13139)

This only includes a newline to ensure there's always something to fold.

Release Notes:

- N/A

crates/assistant/src/slash_command/default_command.rs

@@ -53,7 +53,7 @@ impl SlashCommand for DefaultSlashCommand {
             let prompts = store.default_prompt_metadata();
 
             let mut text = String::new();
-            writeln!(text, "Default Prompt:").unwrap();
+            text.push('\n');
             for prompt in prompts {
                 if let Some(title) = prompt.title {
                     writeln!(text, "/prompt {}", title).unwrap();
@@ -61,6 +61,10 @@ impl SlashCommand for DefaultSlashCommand {
             }
             text.pop();
 
+            if text.is_empty() {
+                text.push('\n');
+            }
+
             Ok(SlashCommandOutput {
                 sections: vec![SlashCommandOutputSection {
                     range: 0..text.len(),