Here there are multiple situations possible.
– Why are there nulls, if the attribute is a PK/FK? Sequence Generator is usually used for PK/FK or the surrogate keys. If that is the case, we need to know why there are nulls in the first place.
– If these are already set as PK/FK/SKs then how did the database load happen with out failure. If these are not set as constraints, these should be set to avoid the missing values.
– The only other case, where it is possible is when data is migrated from some other source or database which does not comply to any of the standards. For this, if its a regular process, then use use the expression transformation to identify the nulls and add a +1 of the maximum sequence available.