workaround for pandas-dev#22305

Author: anmyachev

pandas/_libs/hashtable_class_helper.pxi.in

@@ -1011,14 +1011,19 @@ cdef class PyObjectHashTable(HashTable):
             object val
             khiter_t k
             bint use_na_value
+            bint count_nan = 0
 
         if return_inverse:
             labels = np.empty(n, dtype=np.int64)
         use_na_value = na_value is not None
 
         for i in range(n):
             val = values[i]
-            hash(val)
+            if (val != val or val is None):
+                if count_nan == 0:
+                    uniques.append(val)
+                    count_nan = 1
+                continue
 
             if ignore_na and ((val != val or val is None)
                               or (use_na_value and val == na_value)):