Nodes: Remove "Use Nodes" in Shader Editor for World

Part of https://projects.blender.org/blender/blender/pulls/141278

Blend files compatibility:
If a World exists and "Use Nodes" is disabled, we add new nodes to the
existing node tree (or create one if it doesn't) that emulates the
behavior of a world without a node tree. This ensures backward and
forward compatibility.

Python API compatibility:
- `world.use_nodes` was removed from Python API => **Breaking change**
- `world.color` is still being used by Workbench, so it stays there,
although it has no effect anymore when using Cycles or EEVEE.

Python API changes:
Creating a World using `bpy.data.worlds.new()` now creates a World with
 an empty (embedded) node tree. This was necessary to enable Python
scripts to add nodes without having to create a node tree (which is
currently not possible, because World node trees are embedded).

Pull Request: https://projects.blender.org/blender/blender/pulls/142342

scripts/startup/bl_operators/world.py

@@ -16,7 +16,7 @@ class WORLD_OT_convert_volume_to_mesh(bpy.types.Operator):
     @classmethod
     def poll(cls, context):
         world = cls._world_get(context)
-        if not world or not world.use_nodes:
+        if not world:
             return False
 
         ntree = world.node_tree

scripts/startup/bl_ui/properties_world.py

@@ -120,25 +120,21 @@ class EEVEE_WORLD_PT_surface(WorldButtonsPanel, Panel):
 
         world = context.world
 
-        layout.prop(world, "use_nodes", icon='NODETREE')
         layout.separator()
 
         layout.use_property_split = True
 
-        if world.use_nodes:
-            ntree = world.node_tree
-            node = ntree.get_output_node('EEVEE')
+        ntree = world.node_tree
+        node = ntree.get_output_node('EEVEE')
 
-            if node:
-                input = find_node_input(node, "Surface")
-                if input:
-                    layout.template_node_view(ntree, node, input)
-                else:
-                    layout.label(text="Incompatible output node")
+        if node:
+            input = find_node_input(node, "Surface")
+            if input:
+                layout.template_node_view(ntree, node, input)
             else:
-                layout.label(text="No output node")
+                layout.label(text="Incompatible output node")
         else:
-            layout.prop(world, "color")
+            layout.label(text="No output node")
 
 
 class EEVEE_WORLD_PT_volume(WorldButtonsPanel, Panel):
@@ -151,7 +147,7 @@ class EEVEE_WORLD_PT_volume(WorldButtonsPanel, Panel):
     def poll(cls, context):
         engine = context.engine
         world = context.world
-        return world and world.use_nodes and (engine in cls.COMPAT_ENGINES)
+        return world and (engine in cls.COMPAT_ENGINES)
 
     def draw(self, context):
         layout = self.layout

scripts/startup/bl_ui/space_node.py

@@ -99,7 +99,8 @@ class NODE_HT_header(Header):
 
                 if snode_id:
                     row = layout.row()
-                    row.prop(snode_id, "use_nodes")
+                    if snode.shader_type != 'WORLD':
+                        row.prop(snode_id, "use_nodes")
 
                     if world and world.use_eevee_finite_volume:
                         row.operator("world.convert_volume_to_mesh", emboss=False, icon='WORLD', text="Convert Volume")