fixup! allow fieldcount(T) == 0

Author: vchuravy

src/array.jl

@@ -22,7 +22,7 @@ function explain_eltype(@nospecialize(T), depth=0; maxdepth=10)
           msg *= explain_eltype(U, depth+1)
         end
       end
-    elseif Base.ismutabletype(T)
+    elseif Base.ismutabletype(T) && Base.datatype_fieldcount(T) != 0
       msg = "  "^depth * "$T is a mutable type\n"
     elseif hasfieldcount(T)
       msg = "  "^depth * "$T is a struct that's not allocated inline\n"