Rollback changes done to the InputValidator and the relevant tests

[murtukov]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	yes
Deprecations?	no
Tests pass?	yes
Documented?	no

A PR #1184 changed the validation behavior, effectively making the NotBlank and NotNull constraints not work.

Example:

Person: type: input-object config: fields: firstName: type: String validation: - NotBlank: ~ - NotNull: ~ lastName: type: String validation: - NotBlank: ~ - NotNull: ~

mutation { editPerson( firstName: "John" ) }

This request passes validation, and the lastName field is ignored, even though it is marked NotNull and NotBlank, which is an incorrect behavior.

This PR rolls back changes that introduced this bug.