Guard against calling gemv!
with empty output
#69
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This was made an error in JuliaLang/julia#41513. It would be re-enabled by JuliaLang/julia#42012 or JuliaLang/julia#42054, but obviously, the
gemv!
call can just be omitted if the output is empty anyway. So while this change might turn out not to be required in the long run, it surely doesn't hurt. (And might even buy a little bit of performance for the trivial cases by avoiding unnecessary call overhead while the cost of the extra checks should not matter in non-trivial cases.)