The correct answers are: labels = null, labels is EMPTY, labels in (null) and labels = EMPTY.

Jira Query Language treats missing or absent values as null or EMPTY and all four of the highlighted forms return issues that have no labels. Using labels = null directly checks for a null value. Using labels is EMPTY or labels = EMPTY uses the EMPTY keyword to find fields with no values. Using labels in (null) checks membership against null which is useful when combining the condition with other values in an IN clause.

labels = '' is incorrect because an empty quoted string is not the same as a null or EMPTY value in JQL and it will not reliably match issues that simply have no labels.

labels = "" is incorrect for the same reason and it is not a standard way in JQL to express a missing or empty field.

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