fix bug in import file directives (#117)

the below fails without this change:
```
echo 'val bar = foo' > myScript.sc
echo '//> using file myScript.sc' > test-simple.sc

./srp --script test-simple.sc
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.nio.file.Path.resolve(java.nio.file.Path)" because "base" is null
```

Author: mpollmeier