Update ReleaseNotes.md

.github/workflows/ReleaseNotes.md

 * [Core] Add validation of maximum string lengths (#1718).
 * [Core] Added .NET 4.8 builds (#1729).
 * [Core] Exposed more details of DISCONNECT packet in log (#1729).
+* [Core] Fixed _ArgumentNullException_ in _ConvertPayloadToString_ (#1745, thanks to @ramonsmits).
 * [Client] Added overloads for setting packet payload and will payload (#1720).
 * [Client] The proper connect result is now exposed in the _Disconnected_ event when authentication fails (#1139).
 * [Client] Exposed _UseDefaultCredentials_ and more for Web Socket options and proxy options (#1734, thanks to @impworks).