Fixed NullReferenceException in CommentFormatter.ParseXml

Sometimes during ParseXml it would try calling CommentFormatter.Append and give it null as an argument, which would cause the exception to be thrown. I don't know what causes it to pass null.

Here is the comment that produced the issue:
```cs
/// <summary>
/// When implemented in derived class, processes an event that is not processed
/// anywhere else.
/// </summary>
/// <remarks>
/// Documentation for <see cref="EntityEvent"/> contains details about each event
/// along with descriptions of each parameter.
/// </remarks>
/// <param name="event">           Event to process.</param>
/// <param name="parameter0">      Optional first integer parameter.</param>
/// <param name="parameter1">      Optional second integer parameter.</param>
/// <param name="parameter2">      Optional third integer parameter.</param>
/// <param name="parameter3">      Optional fourth integer parameter.</param>
/// <param name="singleParameter0">
/// Optional first floating point parameter.
/// </param>
/// <param name="singleParameter1">
/// Optional second floating point parameter.
/// </param>
/// <seealso cref="EntityEvent"/>
```
seealso tag seems to be the main offender here: removing it allowed CodeMaid to format everything properly.