[Rd] str2lang throws error when the string is empty

[Dipterix Wang]

Hi, 

str2lang("") raises an error in current version. Would it be good if it returns a missing value expression? One use-case would be to build an expression that subsets an array:

# Expected: x[index1, ]
as.call(list(quote(`[`), quote(x), quote(index1), str2lang("")))

Right now I'm using the following, which is ugly
as.call(list(quote(`[`), quote(x), quote(index1), alist(x=)[[1]]))

Thanks,
- Dipterix
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