Support for ScriptCS files that only contain a #load

If a file only contains #load lines, the `firstCode` variable will be -1, so the file was never loaded. Now this scenario will be supported.

For example, take this file:

`Loader.csx`:

    #load "config.csx"
    #load "program.csx"

This file has no "first code" line, so the load lines were being skipped, resulting in nothing happening.