[server] fix changeRole() when userIDs is empty
[server] fix changeRole() when userIDs is empty

Summary: Changed changeRole() behavior when userIDs array is empty so that instead of throwing SQL syntax error it returns empty Changeset.

Test Plan: Executed function by deleting users on iOS emulator and It worked properly.

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D1961

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