Fix importPart. Refresh imported parts fail when there is no Shape

[mnesarco]

1. I have created two parts from imported SVG files.
2. Then I have created an assembly importing that two parts.
3. Then I have modified the original part files.
4. Then when I click on "Update parts imported into the assembly", the script fails:

Running the Python command 'assembly2_updateImportedPartsCommand' failed:
Traceback (most recent call last):
  File "/home/u/.FreeCAD/Mod/assembly2/assembly2/importPart/__init__.py", line 230, in Activated
    importPart( obj.sourceFile, obj.Name,  doc_assembly )
  File "/home/u/.FreeCAD/Mod/assembly2/assembly2/importPart/__init__.py", line 59, in importPart
    obj_to_copy.Shape =  muxObjects(doc)
  File "/home/u/.FreeCAD/Mod/assembly2/assembly2/utils/muxAssembly.py", line 21, in muxObjects
    faces = faces + obj.Shape.Faces

Fcstd_Property_List instance has no attribute 'Shape'

So I have edited the muxAssembly.py adding two simple checks. It works.

[hamish2014]

Thanks.