Description of problem:
* In ui/gtk/engineabout.py, the existing code assumes self.__engine_desc.icon contains a valid path to the icon file. However, some engines may not have an icon file, hence the variable can be empty. We need to check the variable before using it.

Version-Release number of selected component (if applicable):
* ibus-1.1.0.20090612-3.fc11.x86_64

How reproducible:
* Always

Steps to Reproduce:
1. Run ibus-setup
2. Click on the "Input Methods" tab and select an input method without its own icon
3. Press the "About" button
  
Actual results:
------------------------------------------------------------
Traceback (most recent call last):
  File "/home/hjayasur/ibus-root/share/ibus/setup/main.py", line 201, in __button_engine_about_cb
    about = EngineAbout(engine)
  File "/home/hjayasur/ibus-root/share/ibus/setup/engineabout.py", line 36, in __init__
    self.__init_ui()
  File "/home/hjayasur/ibus-root/share/ibus/setup/engineabout.py", line 50, in __init_ui
    self.__fill_text_view()
  File "/home/hjayasur/ibus-root/share/ibus/setup/engineabout.py", line 59, in __fill_text_view
    text_buffer.insert_pixbuf(iter, self.__load_icon(self.__engine_desc.icon))
  File "/home/hjayasur/ibus-root/share/ibus/setup/engineabout.py", line 92, in __load_icon
    pixbuf = gdk.pixbuf_new_from_file(icon)
glib.GError: Failed to open file '': No such file or directory
------------------------------------------------------------

Expected results:
* Display the "About" window