Improve behaviour of tree creation and Tree::root_id().

`Tree` is now non-empty by default, and `Tree::root_id()` will no longer modify the tree when it is empty. To create an empty tree now it is necessary to use the capacity constructor with a capacity of zero:
  ```c++
  // default-constructed tree is now non-empty
  Tree tree;
  assert(!tree.empty()); // MODIFIED! was empty on previous version
  id_type root = tree.root_id(); // OK. default-constructed tree is now non-empty

  // to create an empty tree:
  Tree tree(0); // pass capacity of zero
  assert(tree.empty()); // as expected
  // but watchout, this is no longer possible:
  //id_type root = tree.root_id(); // ERROR: cannot get root of empty tree.
  ```
This changeset also enables the python library to call `root_id()` on a default-constructed tree

re #556

api/python/tests/test_basic.py

@@ -0,0 +1,13 @@
+import ryml
+
+
+def test_create_empty_tree():
+    tree = ryml.Tree(0)
+    assert tree.empty()
+
+
+def test_create_tree():
+    tree = ryml.Tree()
+    assert not tree.empty()
+    root = tree.root_id()
+    tree.to_seq(root)

api/ryml.i

@@ -597,10 +597,11 @@ using id_type = RYML_ID_TYPE;
 struct Tree
 {
     Tree();
+    Tree(id_type node_type, size_t arena_capacity=RYML_DEFAULT_TREE_ARENA_CAPACITY);
     ~Tree();
 
-    void reserve(id_type node_capacity);
-    void reserve_arena(size_t arena_capacity);
+    void reserve(id_type node_capacity=RYML_DEFAULT_TREE_CAPACITY);
+    void reserve_arena(size_t arena_capacity=RYML_DEFAULT_TREE_ARENA_CAPACITY);
     void clear();
     void clear_arena();