javadoc, started without a java file should display command line options help

javadoc: when you start javadoc without the name of a *.java file for parsing, javadoc
exits with :
javadoc: No packages or classes specified.
java.lang.NullPointerException
at sun.tools.javadoc.Main.error(Main.java:48)
at sun.tools.javadoc.Main.main(Main.java:162)

It should exit gracefully, with some simple command line options help.

Steps to reproduce
run javadoc