mosta doesn't check if a dotOutputFile is specified when validating the parameters

Currently mosta checks whether a dotFormat is specified without an output file which results in an error. It doesn't perform the same check for the dotOutputFile which is also required for the tool to execute without errors. When invoked using the command line this isn't a problem because it is impossible to specify a dot format without also providing a dotOutputFile. However when invoking the tool programmatically it is possible to specify these parameters separately.