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>