Pass empty string not undefined to sessionCallback

Author: cjbj

lib/pool.js

@@ -66,9 +66,10 @@ function completeConnectionRequest(config, getConnectionCb) {
     // Invoke tag fixup callback method if one has been specified and
     // the actual tag on the connection doesn't match the one
     // requested, or the connection is freshly created.
+    let requestedTag = config.tag || "";
     if (typeof self.sessionCallback === 'function' &&
-        (newSession || connInst.tag != config.tag)) {
-      self.sessionCallback(connInst, config.tag,
+        (newSession || connInst.tag != requestedTag)) {
+      self.sessionCallback(connInst, requestedTag,
         function(err) {
           if (err) {
             connInst.close({drop: true}, function() {