Prevent warning when not fully configured

Do not throw a warning when $wgEventLoggingDBname and/or
$wgEventLoggingSchemaApiUri is not set up (a reasonable state in
a wiki which only uses EventLogging as a client, and also the
default setting for MW-Vagrant).

Change-Id: Ib25e6646b60cddfe30542f857e40558e11bf03e8