DefaultParseService: be defensive about null args

It is not allowed to attempt to parse a null value. However, previously,
the exception would come from deep inside SJEP. Let's fail fast instead.

Author: ctrueden

src/main/java/org/scijava/parse/DefaultParseService.java

@@ -60,6 +60,7 @@ public Items parse(final String arg) {
 
 	@Override
 	public Items parse(final String arg, final boolean strict) {
+		if (arg == null) throw new NullPointerException("arg must not be null");
 		return new ItemsList(arg, strict);
 	}