Correctly allocate logical of length 0 in rowwise edge case

[DavisVaughan]

Closes #7710

Really a follow up to #6369

You'll notice that in #6369 we adjusted the code in two places:

• On the R side when the user provides a bare symbol as the expression, which is a special case. We correctly used logical() there https://github.com/tidyverse/dplyr/pull/6369/files#diff-9b0f256680edfcc73a444f94fd9d40fdc00c66105a61d2a3d03331a5938f09e8R342
• On the C side when the user provides any other arbitrary expression. We got that wrong and allocated a logical of length 1 rather than length 0. https://github.com/tidyverse/dplyr/pull/6369/files#diff-d9709c495664bec3d8718ad24bf9db7391406f7ddac818694254efe79b2fe2acR29

Easy fix, easy test!