refactor(data_structures): remove is_empty methods for non-empty stacks (#6219)

Remove `SparseStack::is_empty` method. It's pointless as the stack is never empty.

Add a dummy `NonEmptyStack::is_empty` method that always returns `false`. This is also pointless, but it overrides `slice::is_empty` which is otherwise accessible via `Deref`.

crates/oxc_data_structures/src/stack/non_empty.rs

@@ -294,9 +294,12 @@ impl<T> NonEmptyStack<T> {
         <Self as StackCommon<T>>::len(self)
     }
 
+    /// Get if stack is empty. Always returns `false`.
     #[inline]
     pub fn is_empty(&self) -> bool {
-        self.len() == 0
+        // This method is pointless, as the stack is never empty. But provide it to override
+        // the default method from `slice::is_empty` which is inherited via `Deref`
+        false
     }
 
     /// Get capacity.

crates/oxc_data_structures/src/stack/sparse.rs

@@ -187,15 +187,11 @@ impl<T> SparseStack<T> {
     ///
     /// Number of entries is always at least 1. Stack is never empty.
     #[inline]
+    #[expect(clippy::len_without_is_empty)] // `is_empty` method is pointless. It's never empty.
     pub fn len(&self) -> usize {
         self.has_values.len()
     }
 
-    #[inline]
-    pub fn is_empty(&self) -> bool {
-        self.has_values.len() == 0
-    }
-
     /// Get capacity of stack for any entries (either `Some` or `None`).
     ///
     /// Capacity is always at least 1. Stack is never empty.