Fix building WHERE conditions in query designers

• fixed typo while porting to Kexi 3
• check for null WHERE expression instead of valid expression because validity can't be computed so early

Fix updating SQL statement in SQL designer after switching from other views and property revert to previous SQL

When clearing data cursor while resetting query data view also clear cached data

+ add more checks for existence of data because now it's possible that data is missing

Related: https://bugs.kde.org/377579

Run query designer, set one WHERE condition, switch to data and SQL view, change the condition in the SQL view.
Repeat for 2 or more conditions. Use relations such as = < <= > >= and without relation (defaults to =).

Needed KDb patches: D5294

Note: for 3.0 branches! Will be merged to master when accepted.