YADE should set pre/post-processing variables

Current Situation

• If the source or target directories are not configured (only file_path for example is defined) then the respective ${Source...} or ${Target...} pre/post-processing variables introduced with YADE-448 are empty.
  • the following warning message can be raised:

    main WARN  04:41:02,504   (SOSFileListEntry.java:1443) ::<init> error on resolve path for baseDir=, filePath=C:/Temp/yade_testfile.txt 
    java.lang.IllegalArgumentException: 'other' is different type of Path
    	at sun.nio.fs.WindowsPath.relativize(WindowsPath.java:388)
    	at sun.nio.fs.WindowsPath.relativize(WindowsPath.java:44)
            ...
    

Desired Behaviour

• YADE should correctly set the pre/post-processing variables