Add check if the FailOverPartner key exists first before retrieving its value from the SqlConnectionString (#1614)

Author: lcheunglci

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlConnectionPoolGroupProviderInfo.cs

@@ -104,7 +104,8 @@ private PermissionSet CreateFailoverPermission(SqlConnectionString userConnectio
             //       the server, we will use that name over what was specified  
             //       in the original connection string.
 
-            if (null == userConnectionOptions[SqlConnectionString.KEY.FailoverPartner])
+            if (userConnectionOptions.ContainsKey(SqlConnectionString.KEY.FailoverPartner) &&
+                null == userConnectionOptions[SqlConnectionString.KEY.FailoverPartner])
             {
                 keywordToReplace = SqlConnectionString.KEY.Data_Source;
             }