return immediately from ht_keyindex if dictionary is empty (#44341)

Avoids hashing the input key for empty dictionaries.

Inspired by rust-lang/hashbrown#305.

base/dict.jl

@@ -278,6 +278,7 @@ end
 
 # get the index where a key is stored, or -1 if not present
 function ht_keyindex(h::Dict{K,V}, key) where V where K
+ isempty(h) && return -1
  sz = length(h.keys)
  iter = 0
  maxprobe = h.maxprobe