(fs) FileSystems.getFileSystem(URI) should throw IAE if the URI scheme is null

If the scheme is null, FileSystems.getFileSystem(URI) will currently throw a mysterious NullPointerException. The package documentation for java.nio.file indicates that NPEs will in general be thrown if a parameter is null or an array or collection contains a null element, but this case does not meet those conditions. Also, using the URI as the detail message of an IAE could provide debugging clues.