`is_grounded_call` doesn't handle absence of slot types properly

right now it returns true which is crazy. Should throw an error.

UPD 2023-04-26: perhaps an actionable direction would be to try to reproduce it by removing the following code from is_grounded_call and run the analysis on the packages to see what happens...

    # the following check is taken verbatim from code_warntype
    if !isa(slottypes, Vector{Any})
        return true # I don't know when we get here,
                    # over-approx. as stable
    end

prl-julia / julia-type-stability

`is_grounded_call` doesn't handle absence of slot types properly #14