fix: don't add empty query string items to connection string

In the legacy parser, if no query string items are parsed from a
TXT record look up, no items should be added to the second parse

lib/url_parser.js

@@ -99,13 +99,16 @@ module.exports = function(url, options, callback) {
 
         record = record[0].join('');
         const parsedRecord = qs.parse(record);
-        if (Object.keys(parsedRecord).some(key => key !== 'authSource' && key !== 'replicaSet')) {
+        const items = Object.keys(parsedRecord);
+        if (items.some(item => item !== 'authSource' && item !== 'replicaSet')) {
           return callback(
             new MongoParseError('Text record must only set `authSource` or `replicaSet`')
           );
         }
 
-        connectionStringOptions.push(record);
+        if (items.length > 0) {
+          connectionStringOptions.push(record);
+        }
       }
 
       // Add any options to the connection string