Apologies if I am repeating stuff you already know. The database standard is to distinguish ‘NULL’ and blanks – NULL is <Unknown> and a blank represents data that is valid and blank. Access and SQL Server, unlike some database systems, behave themselves in this respect and distinguish between the two. So it would be wrong in principle to change the Null data to blank, since Nulls should never match other Null fields in a search.
For example, you would not want a blood bank to match a sample with Null blood type (not yet entered) to a patient with Null blood type (not yet identified).
However, in practice these are sometimes blurred and if you genuinely want to match both Nulls and blanks, can your team program against a view rather than a table, where the field(s) in question are replaced by calculated fields, e.g. ISNULL(<FieldThatMayBeNull>,””)?
This way the original data remains unchanged and you only combine the Nulls and blanks where appropriate.