GetConverter never returns null (dotnet#8444)

Author: gewarren

xml/System.Text.Json/JsonSerializerOptions.xml

@@ -429,7 +429,7 @@ For more information, see [How to customize character encoding with System.Text.
       <Docs>
         <param name="typeToConvert">The type to return a converter for.</param>
         <summary>Returns the converter for the specified type.</summary>
-        <returns>The first converter that supports the given type, or <see langword="null" /> if there is no converter.</returns>
+        <returns>The first converter that supports the given type.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[