Hi @kamlesh_vaghela
the documentation states:
Description
Replaces null values with a specified value. Null values are field values that are missing in a particular result but present in another result. Use fillnull to replace null field values with a string. If you do not specify a field list, fillnull replaces all null values with 0 (the default) or a user-supplied string.
The last sentence in bold is interesting.
But your example is also quite good, which leads me to believe that either the documentation is unclear on this, and should be updated, or, this is a bug.
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