The spec
http://docs.oracle.com/javase/8/docs/api/javax/script/ScriptEngineFactory.html#getParameter-java.lang.String-
says:
"Returns null if no value is assigned to the key."
and doesn't mention any NPEs.

However the following line executed on Oracle's JDK.

new ScriptEngineManager().getEngineFactories().forEach(f -> f.getParameter(null));

will throw

java.lang.NullPointerException
at jdk.nashorn.api.scripting.NashornScriptEngineFactory.getParameter(NashornScriptEngineFactory.java:107)

which doesn't perfectly conform to the current spec and the corresponding JCK9(b05) test will fail on JDK9:

api/javax_script/ScriptEngineFactory/index.html#GetParameter[nullKey]

The specification or the Nashorn implementation needs to be corrected.