UpdateOrders param default orderset = NULL

I am calling `updateOrders` with the `orderset` param left as default which is `NULL`. Line 524 of `orders.R` in `updateOrders` is calling `is.na()` on the `orderset` param which is causing an `argument is of length zero` error.

Specifically in my case, I am calling `updateOrders` from the `ruleRevoke` function. If I set `orderset = NA` then I no longer get the error.

Min Reproducible example is:
`add.rule(strategy.st, name="ruleRevoke", arguments=list(sigcol="EOD", sigval=TRUE, portfolio = portfolio.st, symbols = symbols, ruletype = "risk"), type="risk", enabled=TRUE)` where `"EOD"` is a signal column which has a value of TRUE at the End Of Day.

Start in orders.R at line 524 within updateOrders, then inspect the ruleRevoke call path described in the issue. Run the provided add.rule reproduction with orderset omitted and with orderset set to NA, and confirm that the default case no longer raises the reported error while the existing NA case still works.

Written by the indexing model from the issue text.