UI: Only draw node menu search if categories exist

Currently, the search operator in the node add menu NODE_MT_add is drawn
even if no node categories exists. This patch only draws the operator if
at least one node category passes the poll.

This patch is needed because some add-ons use custom search operator and
do not register node categories. In this case, it is undesirable to have
a search operator drawn that do nothing and is not used. One such add-on
is Animation Nodes.

Reviewed By: Jacques Lucke

Differential Revision: https://developer.blender.org/D8576

release/scripts/modules/nodeitems_utils.py

@@ -144,6 +144,14 @@ def node_categories_iter(context):
                 yield cat
 
 
+def has_node_categories(context):
+    for cat_type in _node_categories.values():
+        for cat in cat_type[0]:
+            if cat.poll and ((context is None) or cat.poll(context)):
+                return True
+    return False
+
+
 def node_items_iter(context):
     for cat in node_categories_iter(context):
         for item in cat.items(context):