Fix T58094: Failure to import .obj files without material data.

Not sure how we did this in old code, but we certainly cannot ignore
None (a.k.a. default place holder) material anymore... Do not generate
it when not needed, though!

io_scene_obj/__init__.py

@@ -21,7 +21,7 @@
 bl_info = {
     "name": "Wavefront OBJ format",
     "author": "Campbell Barton, Bastien Montagne",
-    "version": (3, 5, 1),
+    "version": (3, 5, 2),
     "blender": (2, 80, 0),
     "location": "File > Import-Export",
     "description": "Import-Export OBJ, Import OBJ mesh, UV's, materials and textures",

io_scene_obj/import_obj.py

@@ -94,6 +94,7 @@ def create_materials(filepath, relpath,
     assign colors and images to the materials from all referenced material libs
     """
     from math import sqrt
+    from bpy_extras import node_shader_utils
 
     DIR = os.path.dirname(filepath)
     context_material_vars = set()
@@ -176,21 +177,19 @@ def create_materials(filepath, relpath,
         else:
             raise Exception("invalid type %r" % type)
 
-    # Add an MTL with the same name as the obj if no MTLs are spesified.
+    # Try to find a MTL with the same name as the OBJ if no MTLs are specified.
     temp_mtl = os.path.splitext((os.path.basename(filepath)))[0] + ".mtl"
-
     if os.path.exists(os.path.join(DIR, temp_mtl)):
         material_libs.add(temp_mtl)
     del temp_mtl
 
     # Create new materials
     for name in unique_materials:  # .keys()
-        if name is not None:
-            ma = unique_materials[name] = bpy.data.materials.new(name.decode('utf-8', "replace"))
-            from bpy_extras import node_shader_utils
-            ma_wrap = node_shader_utils.PrincipledBSDFWrapper(ma, is_readonly=False)
-            nodal_material_wrap_map[ma] = ma_wrap
-            ma_wrap.use_nodes = True
+        ma_name = "Default OBJ" if name is None else name.decode('utf-8', "replace")
+        ma = unique_materials[name] = bpy.data.materials.new(ma_name)
+        ma_wrap = node_shader_utils.PrincipledBSDFWrapper(ma, is_readonly=False)
+        nodal_material_wrap_map[ma] = ma_wrap
+        ma_wrap.use_nodes = True
 
     for libname in sorted(material_libs):
         # print(libname)
@@ -479,7 +478,7 @@ def split_mesh(verts_loc, faces, unique_materials, filepath, SPLIT_OB_OR_GROUP):
 
             face_vert_loc_indices[loop_idx] = map_index  # remap to the local index
 
-            if context_material and context_material not in unique_materials_split:
+            if context_material not in unique_materials_split:
                 unique_materials_split[context_material] = unique_materials[context_material]
 
         faces_split.append(face)
@@ -902,6 +901,7 @@ def load(context,
         context_parm = b''  # used by nurbs too but could be used elsewhere
 
         # Until we can use sets
+        use_default_material = False
         unique_materials = {}
         unique_smooth_groups = {}
         # unique_obects= {} - no use for this variable since the objects are stored in the face.
@@ -982,6 +982,8 @@ def load(context,
                         verts_loc_len = len(verts_loc)
                         verts_nor_len = len(verts_nor)
                         verts_tex_len = len(verts_tex)
+                        if context_material is None:
+                            use_default_material = True
                     # Else, use face_vert_loc_indices and face_vert_tex_indices previously defined and used the obj_face
 
                     context_multi_line = b'f' if strip_slash(line_split) else b''
@@ -1152,6 +1154,8 @@ def load(context,
 
         progress.step("Done, loading materials and images...")
 
+        if use_default_material:
+            unique_materials[None] = None
         create_materials(filepath, relpath, material_libs, unique_materials,
                          use_image_search, float_func)