Let the basic dialog actions be available for empty value context by default

Unlike pre-6.2 dialogs, updated implementation applies availability check to the dialog actions by default. Apart from other implications, this also means the the dialogs actions have to be explicitly configured to be compatible with an empty values context case (aka root: true), which might not be convenient (and not expected by many).

Proposed solution: let the default dialog actions (commit/cancel) have pre-configured availability definition with root property set to true. In case the user decides to have explicit availability configuration - they'd have to also configure the mentioned property explicitly.