[Reference: https://answers.splunk.com/answers/653427/fillnull-doesnt-work-without-specfying-a-field.html, ,  The fillnull command is a search command that replaces null values with a specified value or 0 if no value is specified. Null values are values that are missing, empty, or undefined in Splunk. The fillnull command can replace null values for all fields or for specific fields. The fillnull command can take an optional argument called value that specifies the value to replace null values with. If no value argument is specified, the fillnull command will replace null values with 0 by default., , , , , , ]