bug: safely set window class and instance
bug: safely set window class and instance

In some cases, the data will have the `window_properties` object but
class or instance will not be set, such as is the case with i3-nagbar,
which would cause a crash when accessing the property.

chrsclmn/i3ipc-python,nicoe/i3ipc-python,acrisci/i3ipc-python,xenomachina/i3ipc-python