rxSummary transforms(n=1) fails if call has no references to existing vars

[hongooi73]

These fail:

mtx <- rxDataStep(mtcars, "mtcars.xdf", overwrite=TRUE)

rxSummary(~n, mtx, transforms=list(n=1L))
rxSummary(~n, mtx, transforms=list(n=rep(1L, .rxNumRows)))

These work:

rxSummary(~n, mtx, transforms=list(n=1L, m=mpg))  # throwaway variable
rxSummary(~n, mtx, transforms=list(n=rep(1L, length(mpg))))
rxSummary(~n:F(am), mtx, transforms=list(n=1L))

[hongooi73]

But this also fails:

rxSummary(~n:F(am), mtx, transforms=list(n=rep(1, .rxNumRows)))

[hongooi73]

TFS item 78067