chore: Fix several style lints (#17488)

It's not comprehensive enough to start linting on `style` group, but
hey, it's a start.

Release Notes:

- N/A

crates/welcome/src/base_keymap_setting.rs

@@ -79,7 +79,7 @@ impl BaseKeymap {
         Self::OPTIONS
             .iter()
             .copied()
-            .find_map(|(name, value)| (name == option).then(|| value))
+            .find_map(|(name, value)| (name == option).then_some(value))
             .unwrap_or_default()
     }
 }

crates/welcome/src/multibuffer_hint.rs

@@ -18,6 +18,12 @@ const NUMBER_OF_HINTS: usize = 10;
 
 const SHOWN_COUNT_KEY: &str = "MULTIBUFFER_HINT_SHOWN_COUNT";
 
+impl Default for MultibufferHint {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl MultibufferHint {
     pub fn new() -> Self {
         Self {