SQLite: add NOT NULL constraints where applicable

[link2xt]

Currently sql.rs create tables without any NOT NULL constraints. Adding them could avoid adding NULLs by accidentally passing None as SQL query param. See #283 (comment)

[Jikstra]

Very good idea, i think this could help us figuring out where we accidentally put null into the db which could be related to those issues:
#223
#233

[hpk42]

sql-not-null -- we are generally more careful now (doing to_string_lossy everywhere etc.) and in several places we use "?" bubbling up errors. the original issues referenced on aug4 from @Jikstra are fixed already. So i suggest that instead of keeping this as issue just do a PR on some further SQL-processing.