Don't signal a backtrace on empty --script files

Don't signal a backtrace on empty --script files

* lisp/startup.el (command-line--load-script): New function that
avoids erroring out if it turns out there's no forms in the buffer
(bug#4616).

* lisp/subr.el (delete-line): New utility function.
* lisp/international/mule.el (load-with-code-conversion): Accept
an eval function.