Remember that if you set the Null value property in metadata for any string data field to any non-empty string, any function that accept string data field as an argument and throws NPE when applied on null (e.g., length()), it will throw NPE when applied on such specific string.

For example, if field1 has Null value property set to "<null>", length($0.field1) will fail on the records in which the value of field1 is "<null>" and it will be 0 for empty field.

See Null value for detailed information.