jvb: fix check for JVB_TCP_HARVESTER_DISABLED

Fixes: https://github.com/jitsi/docker-jitsi-meet/issues/885

jvb/rootfs/defaults/jvb.conf

@@ -10,7 +10,7 @@ videobridge {
             port = {{ .Env.JVB_PORT }}
         }
         tcp {
-            enabled = {{ not .Env.JVB_TCP_HARVESTER_DISABLED }}
+            enabled = {{ not (.Env.JVB_TCP_HARVESTER_DISABLED | default "true" | toBool) }}
             port = {{ .Env.JVB_TCP_PORT }}
 {{ if not (eq $JVB_TCP_PORT $JVB_TCP_MAPPED_PORT) }}
             mapped-port = {{ $JVB_TCP_MAPPED_PORT }}