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Where not(source.field) is null
does not make any sort of sense to me

Where not source.field is null

Or

Where not (source.field is null)

in proper SQL you might find a use for the NullIf function which is a sort of inverse of IsNull

NullIf(Source.Field,0)

useful if someone in the past got round a nullability problem by putting rubbish in the field.