addPolymorphicFields(SObjectField, Map<SObjectType, List<>>) with empty list in the map results in a malformed query

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System.debug(
	new Query(Task.SObjectType)
		.addPolymorphicFields(
			Task.WhoId,
			new Map<SObjectType, List<SOQL.QueryField>>{
				Contact.SObjectType => new List<SOQL.QueryField>{}
			}
		)
		.getQuery()
);

When addPolymorphicFields(SObjectField, Map<SObjectType, List<SObjectField>>) or addPolymorphicFields(SObjectField, Map<SObjectType, List<SOQL.QueryField>>) is used with a non-empty map, but with empty list as one of the values, the resulting query literally queries no fields for that SObject type (notice the double space after THEN):

SELECT Id, Subject, TYPEOF Who WHEN Contact THEN  ELSE Alias, Email, (...) END FROM Task

Please consider throwing a runtime exception for an empty list or, arguably better, adding Id as the default field if none are specified:

SELECT Id, Subject, TYPEOF Who WHEN Contact THEN Id ELSE Alias, Email, (...) END FROM Task