That makes an unreasonable assumption that everybody across the whole industry is not just competent and can be trusted to have it together, but they also manage to achieve their important goals (which are not always having the cleanest code or architecture) using the same architectural dogmas as you do. One can define a datatype as FormattedValue | null, make a tuple of (value, present), have a special empty value or do all of that at once, because parts of the dataset were separately inherited during the merger.
I'm not saying it doesn't happen - I'm saying it's not the correct practice. When talking about financial services, this sort of garbage data could lead to compliance issues and financial penalties. Typically those mergers end up with remediation initiatives to at least ensure that all the invalid data is nulled out, or facilitate it's collection if it's critical.