Change whatsnew for changes

Author: ssikdar1

doc/source/whatsnew/v0.23.1.txt

@@ -45,6 +45,7 @@ Bug Fixes
 ~~~~~~~~~
 
 - tab completion on :class:`Index` in IPython no longer outputs deprecation warnings (:issue:`21125`)
+- fix normalize to not except when json_normalize is fed a key with null value on inner most key (:issue:`21158`)
 
 Groupby/Resample/Rolling
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