gives 4 as the answer (or whatever the rounding value is). Usually, in
the situation where this arose, \l_tmpa_tl is a string of digits
possibly including a decimal point. In this instance it was empty. Okay,
I can check for that, but I wonder if l3fp should do so, and either give
an error, as
\fp_set:Nn \l_tmpa_fp { \l_tmpa_tl }
does when the token list is empty, or give a conventional 0 as answer?