rust: rbtree: add RBTree::is_empty

In Rust Binder I need to be able to determine whether a red/black tree
is empty. Thus, add a method for that operation to replace

	rbtree.iter().next().is_none()

This is terrible, so add a method for this purpose. We do not add a
RBTree::len method because computing the number of elements requires
iterating the entire tree, but checking whether it is empty can be done
cheaply.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250616-rbtree-is-empty-v1-1-61f7cfb012e3@google.com
[ Adjusted title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Darksonn

rust/kernel/rbtree.rs

@@ -191,6 +191,12 @@ impl<K, V> RBTree<K, V> {
         }
     }
 
+    /// Returns true if this tree is empty.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.root.rb_node.is_null()
+    }
+
     /// Returns an iterator over the tree nodes, sorted by key.
     pub fn iter(&self) -> Iter<'_, K, V> {
         Iter {