(Background is issue FreeCAD#18622)

CMake errors out when the Kuka files are removed with:

```
CMake Error at cMake/FreeCadMacros.cmake:79 (ADD_CUSTOM_COMMAND):
  ADD_CUSTOM_COMMAND called with wrong number of arguments.
Call Stack (most recent call first):
  src/Mod/Robot/CMakeLists.txt:47 (fc_target_copy_resource)
```

The problem is that while the CMake code checks whether /src/Mod/Robot/Lib/Kuka is there befor setting Robot_Resources, but then later still uses the variable, even if it hasn't been set.

The patch just guards the failing fc_target_copy_resource with another if that checks whether the variable has been defined.

CMake install would also fail when Lib is empty, so another guard is required for the `INSTALL( DIRECTORY Lib` … section.