MB-5307: prevent spamming of logs by useless empty xdcr stats
MB-5307: prevent spamming of logs by useless empty xdcr stats

Change-Id: Ia0635b83728fc61debd453ada7a5af18a93c13ce
Reviewed-on: http://review.couchbase.org/18214
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

membase/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,membase/ns_server,ceejatec/ns_server,mschoch/ns_server,vzasade/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,membase/ns_server,t3rm1n4l/ns_server,membase/ns_server,mschoch/ns_server,daverigby/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,daverigby/ns_server,mschoch/ns_server,membase/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,nimishzynga/ns_server,ceejatec/ns_server,mschoch/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,mschoch/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,ceejatec/ns_server,vzasade/ns_server,vzasade/ns_server,membase/ns_server