fix issue getting styles if object had empty mat slots

Occurred only with only_assigned_to_faces = True
A bit related to #4675

Error was
    styles = [style for style, usage in zip(styles, usage_count, strict=True) if usage > 0]

ValueError: zip() argument 2 is longer than argument 1

src/blenderbim/blenderbim/tool/geometry.py

@@ -470,8 +470,15 @@ class Geometry(blenderbim.core.tool.Geometry):
         usage_count = [0] * len(obj.material_slots)
         if not usage_count:  # if there are no materials, polygons will still use index 0
             return []
+
         for poly in obj.data.polygons:
             usage_count[poly.material_index] += 1
+
+        # remove usages for empty material slots
+        for i, slot in reversed(list(enumerate(obj.material_slots))):
+            if not slot.material:
+                del usage_count[i]
+
         styles = [style for style, usage in zip(styles, usage_count, strict=True) if usage > 0]
         return styles