Better handling validators - added checks to avoid not intended usage

[mproch]

e.g.
'' + 'dd'
is considered blank....

[arkadius]

In fact, this validator like others e.g. min/max and so on for now only works with literal expressions (eager parameters) and IMO there is no need to make it work with lazy parameters. Work that should be done is:

• Add documentation - maybe here: https://nussknacker.io/API.html ?
• Add check that validation annotation is not added to lazy parameters - maybe also check that you've added editor that validator is intented to work with