Fix logic bug introduced in #14730 (#14742)

The removal of `valid_count` on columns in #14730 had one logic bug, fixed here.

Authors:
  - Lawrence Mitchell (https://github.com/wence-)

Approvers:
  - Matthew Roeschke (https://github.com/mroeschke)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #14742

python/cudf/cudf/core/column/categorical.py

@@ -1380,7 +1380,7 @@ def _concat(
 
         # Find the first non-null column:
         head = next(
-            (obj for obj in objs if not obj.null_count != len(obj)), objs[0]
+            (obj for obj in objs if obj.null_count != len(obj)), objs[0]
         )
 
         # Combine and de-dupe the categories