MicroBat cannot run unit tests in default package

The following line 222 in microbat.util.MicroBatUtil , `cName = cName.substring(0, cName.lastIndexOf(".")).replace(".", File.separator);` throws String index out of bounds exception when the class is not in any package. Specifically `cName.substring(0, cName.lastIndexOf("."))` as it resolves to `cName.substring(0, -1)`
