Fix invalid preprocessing on Keeper

[antonio2368]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Keeper fix: Leader should correctly fail on preprocessing a request if it is not initialized.

Previously, leader node would simply return default Ok if it's not initialized which means that some request info is not correctly set (like ZXID).

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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