Fix T53254: Fbx import assertion error on some Enum custom property.

Do not try to get some string namecode of Enum items if string part of
the custom FBX Enum property is empty! Just stick to basic int value in
this case.

io_scene_fbx/__init__.py

@@ -21,7 +21,7 @@
 bl_info = {
     "name": "FBX format",
     "author": "Campbell Barton, Bastien Montagne, Jens Restemeier",
-    "version": (3, 8, 3),
+    "version": (3, 8, 4),
     "blender": (2, 79, 1),
     "location": "File > Import-Export",
     "description": "FBX IO meshes, UV's, vertex colors, materials, textures, cameras, lamps and actions",

io_scene_fbx/import_fbx.py

@@ -343,7 +343,7 @@ def blen_read_custom_properties(fbx_obj, blen_obj, settings):
                     elif prop_type in {b'Enum', b'enum'}:
                         assert(fbx_prop.props_type[4:6] == bytes((data_types.INT32, data_types.STRING)))
                         val = fbx_prop.props[4]
-                        if settings.use_custom_props_enum_as_string:
+                        if settings.use_custom_props_enum_as_string and fbx_prop.props[5]:
                             enum_items = fbx_prop.props[5].decode('utf-8').split('~')
                             assert(val >= 0 and val < len(enum_items))
                             blen_obj[prop_name] = enum_items[val]