Turn on Typescript strictNullChecks rule

## Now
Historically `strictNullChecks` is set to `false`

## To Do
- Set `strictNullChecks: 'true'` for all uui packages and root tsconfig package(for DEV build).
- Fix all appeared errors

## Describe the solution you'd like (optional)
It seams that we would have a lot of errors and probably it will be not manageable to fix them all in a row. Probably we can turn on this rule separately for each package and fix errors package by package. But until we turn it on for each package we will can't turn on it for DEV mode and it will lead to difference between dev and prod build.