Bugfix for Jira item THRONE-50

Some minor changes to address Bugfix for Jira item THRONE-50: Download Exported Job fails.

When ftp configuration is not set in Config.groovy, a MissingMethodException is thrown in PostgresExportService. To fix this I've covered both bases:

1) By adding in lines in the config with empty strings set for the appropriate settings, the defs that read them in PostgresExportService get correctly cast.
2) I have re-typed the variables that read in the config strings so that if they are not present in the config file at all, the excepted types are still passed to exportDataProcessor.getExportJobFileStream() method making the method signature correct.