mutate() with functions that return NULL

[krlmlr]

gives an error. Is this by design? I can't find a test for this.

mutate(iris, x = identity(NULL))
## Error in eval(substitute(expr), envir, enclos) : 
##   Unsupported type NILSXP for column "x"
mutate(iris, x = NULL)
## (works as expected)

[krlmlr]

Return sensible error message.

[hadley]

We do need to handle NULL returned from functions somehow. Let's discuss in the future.