Fixes jruby#7918. Error with single splat method with keyword arguments

Problem was that we were seeing an empty kwarg marked from
ruby2_keyword_hash and omitting it because it was a kwarg.  The
logic doing this was for a failing test in MRI test suite that
was passing that empty kwarg using a splat.  The fix was to
only pass this along if it is a non-splat caller.

core/src/main/java/org/jruby/ir/runtime/IRRuntimeHelpers.java

@@ -790,7 +790,7 @@ public static IRubyObject receiveKeywords(ThreadContext context, IRubyObject[] a
 
             args[args.length - 1] = hash;
             return UNDEFINED;
-        } else if (ruby2_keywords_hash && hash.isEmpty()) {
+        } else if ((callInfo & CALL_SPLATS) != 0 && ruby2_keywords_hash && hash.isEmpty()) {
             // case where we somehow (hash.clear) a marked ruby2_keyword.  We pass it as keyword even in non-keyword
             // accepting methods so it is subtracted from the arity count.  Normally empty keyword arguments are not
             // passed along but ruby2_keyword is a strange case since it is mutable by users.