[keyserver] Introduce isCookieMissingSignedIdentityKeysBlob

Summary:
Checks if there's an existing row in cookies table with id=cookieID with a NULL signed_identity_keys column.

We're going to use this check in handleInitialClientSocketMessage to determine whether or not to send SIGNED_IDENTITY_KEYS_BLOB request to client from keyserver.

Depends on D7049

Test Plan:
Called function from handleInitialClientSocketMessage:

if (viewer.cookieID) {
  isCookieMissingSignedIdentityKeysBlob(viewer.cookieID);
}

and set breakpoint within isCookieMissingSignedIdentityKeysBlob to check whether queryResult was as expected when signed_identity_keys was NULL and a valid stringified SignedIdentityKeysBlob:

Reviewers: ashoat

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D7050