Backport: fix varargs (i.e. <n+:n>) signature validation (#319)

Using <n+:n> signature, validate will return the first arg n times.
As we're based on this impl the fix should be backported

Original fix here:
dashjoin/jsonata-java@3849b71

Test case:
dashjoin/jsonata-java@abf12e9

Author: uw4

src/main/java/com/api/jsonata4java/Signature.java

@@ -383,6 +383,7 @@ ArrayNode validate(String functionName, ExprListContext args, ExpressionsVisitor
                             validatedArgs.add(arg);
                             argIndex++;
                         } else {
+                            arg = expressionVisitor.visit(args.expr(argIndex));
                             validatedArgs.add(arg);
                             argIndex++;
                         }