fix sysimage-native-code=yes option (#53407)
Follow up to #53373, it seems this assert was broken for empty packages,
causing CI issues. It is not necessary.
Observed in CI here: https://github.com/JuliaLang/julia/pull/53395
https://buildkite.com/julialang/julia-master/builds/33860#018dc4dc-a603-4ad1-90cf-574540a41720
