UI: Compositor: Show Parent button when available

The Parent Node Tree operator button to go up a level in nested
node trees is always visible in the header, regardless if we are
currently in a nested tree or not.

Only show the button when it is actually possible to go a level up.
This frees up space in the header when at the top level.

scripts/startup/bl_ui/space_node.py

@@ -197,7 +197,8 @@ class NODE_HT_header(Header):
         if is_compositor:
             layout.prop(snode, "pin", text="", emboss=False)
 
-        layout.operator("node.tree_path_parent", text="", icon='FILE_PARENT')
+        if len(snode.path) > 1:
+            layout.operator("node.tree_path_parent", text="", icon='FILE_PARENT')
 
         # Backdrop
         if is_compositor: