materials ui - avoid having multiple "Uncategorised" categories

Previously it would create multiple same named "Uncategorised" categories for each case when Category was "", None, "Uncategorised", which was confusing and some of them ("" and None) would expand simultaneously when you would try to expand one.

Removed `or "Uncategorised"` from ui.py as there shouldn't be a need for this since all cases are handled when items are added.

src/blenderbim/blenderbim/bim/module/material/ui.py

@@ -343,7 +343,7 @@ class BIM_UL_materials(UIList):
                     row.operator(
                         "bim.expand_material_category", text="", emboss=False, icon="DISCLOSURE_TRI_RIGHT"
                     ).category = item.name
-                row.label(text=item.name or "Uncategorised")
+                row.label(text=item.name)
             else:
                 row.label(text="", icon="BLANK1")
                 row.label(text=item.name, icon="MATERIAL")

src/blenderbim/blenderbim/tool/material.py

@@ -24,6 +24,7 @@ import blenderbim.tool as tool
 import blenderbim.bim.helper
 import ifcopenshell.util.unit
 import ifcopenshell.util.element
+from collections import defaultdict
 from typing import Union, Any
 
 
@@ -67,12 +68,16 @@ class Material(blenderbim.core.tool.Material):
         elif material_type == "IfcMaterialList":
             get_name = lambda x: "Unnamed"
         materials = sorted(tool.Ifc.get().by_type(material_type), key=get_name)
-        categories = {}
+        categories = defaultdict(list)
         if material_type == "IfcMaterial":
-            [categories.setdefault(getattr(m, "Category", "Uncategorised"), []).append(m) for m in materials]
+            for m in materials:
+                # IfcMaterial has Category since IFC4.
+                category = getattr(m, "Category", None)
+                category = category or "Uncategorised"
+                categories[category].append(m)
             for category, mats in categories.items():
                 cat = props.materials.add()
-                cat.name = category or ""
+                cat.name = category
                 cat.is_category = True
                 cat.is_expanded = cat.name in expanded_categories
                 for material in mats if cat.is_expanded else []: