Resolution: Won't Fix
(NB: I would set the priority to P2 default to be triaged, but I seem not to have that option, so I left it at the default I was presented with.)
When calling a method with a varargs parameter with a single value, that value is wrapped into an array of length 1. This is the behaviour in Java, and is the expected behaviour, and most of the time is the behaviour in Groovy too.
But when that single value is null, Groovy will instead just pass the null into the method. Java will not do this: You'll get an array with a single null in it. Because Groovy's behaviour is unexpected, especially when interfacing with Java code, NullPointerExceptions can often ensue.
Adding to the inconsistencies, if the Groovy code calling the method is in a @CompileStatic context, it behaves just like Java, and the method (whether or not it is statically compiled or a dynamic Groovy method) receives an array with a null in it.
So the behaviour in Groovy is inconsistent, both with itself in a @CompileStatic situation, and with Java, and is requiring workarounds in Java code to handle this normally-impossible eventuality. (Even if no varargs parameter is given you get an empty array, as in fact you do in Groovy too.)
This may be an "early instalment weirdness": There's an ancient ticket, from not long after varargs were introduced into Java, which appears to have argued - successfully at the time - that the normal behaviour is a bug: https://issues.apache.org/jira/browse/GROOVY-1026
Further adding to the confusion may be that Groovy usually elides the difference between an Object parameter and an Object... parameter: They both behave the same.
The offending code appears to be in org.codehaus.groovy.reflection.ParameterTypes.java in method fitToVars, lines 200-215 in master at the time of writing, which even includes a comment that "if the last argument is null, then we don't have to do anything", with which I respectfully disagree. That behaviour should be to return an array with a single null in it (Handily, MetaClassHelper.ARRAY_WITH_NULL saves having to make a new one.)
In principle it's an easy fix (although I've left tagging to others as this is my first issue here), but there'd be an obvious nervousness about changing behaviour like this when there might be a lot of old code out there depending on it behaving the way it does now. OTOH the way it behaves now is breaking the expectations of those of us coming to Groovy from a lifetime of Java...
VarArgsTest.groovy - a script saved from, and runnable in, groovyConsole, demonstrating the behaviour. The behaviour is the same regardless of whether the console is launched with the -indy option. (The issue was initially observed in indy.) The dynamic portion of the test, when run, ends in a NullPointerException as Arrays.asList is not expecting a null varargs parameter. Output seen (-indy or not):
(etc. stack trace not shown for formatting reasons.)
VarArgsTest.jsh - a jshell script demonstrating Java's behaviour, very similar to the groovy test, but omitting the dynamic portion of the test for obvious reasons. (The statements in the Groovy script ending in semicolons are left that way precisely to mark that they're identical to the Java test.) Runnable with