[PowerShell] Fix missing Content-Type header on a 204 response (#19340)

* Allow for binary response content

* AllowNull for $ContentTypes

* Update PSApiClient.ps1

samples/client/echo_api/powershell/src/PSOpenAPITools/Private/ApiClient.ps1

@@ -223,6 +223,7 @@ function DeserializeResponse {
         [Parameter(Mandatory)]
         [Object]$Response,
         [Parameter(Mandatory)]
+        [AllowNull()]
         [AllowEmptyCollection()]
         [string[]]$ContentTypes
     )