Allow empty paths in FileInputFormat to fix a problem with the job-preview

The commit [CsvFormat configurable via regular parameters](https://github.com/stratosphere/stratosphere/commit/9dd4635ad69bf107ab9f81c8e7ffd495475471e0) caused the job-preview of the job-submission web interface to fail. The preview currently creates a plan with empty String args. This is no longer possible due to this commit (see my changes to understand).

I added a quick-fix (see code, very simple), and updated a test case. Please merge for Ufuk.