Fix new upgrade script

Author: lgoettgens

src/Serialization/Upgrades/1.6.0-1.jl

@@ -1,11 +1,11 @@
 push!(upgrade_scripts_set, UpgradeScript(
   v"1.6.0-1",
-  function upgrade_1_6_0_1(s::UpgradeState, dict::Dict)
+  function upgrade_1_6_0_1(s::UpgradeState, dict::AbstractDict{Symbol, Any})
     # recurse upgrade on containers
     upgrade_containers(upgrade_1_6_0_1, s, dict)
 
     # Upgrades
-    if dict[:_type] isa Dict && get(dict[:_type], :name, "") == "RationalFunctionField"
+    if dict[:_type] isa AbstractDict && get(dict[:_type], :name, nothing) == "RationalFunctionField"
       if dict[:data][:symbols] isa String
         dict[:data][:symbol] = dict[:data][:symbols]
         delete!(dict[:data], :symbols)