broken video stream cannot be deleted

[lemmitkaplinski-png]

After some testing I ended up with a video stream that has no path. Don't ask me how, but it happened.

This stream cannot be deleted from the UI. The error is: "Failed to delete video stream Please specify a path name"

In order to delete this stream, I have to manually perform an SQL query: "DELETE from video_streams WHERE path='';"

Expected behaviour:

1. Do not allow creating streams with no path.
2. Do not use path as unique identifier, use the uid that is stored in SQL.