Reflection: Fix DefaultValue for optional DateTime (#17877)

Querying the default value of an optional `DateTime` parameter using
`ParameterInfo.DefaultValue` can throw a `FormatException` if that
default value is a null constant in metadata, which is how the C#
compiler encodes a default value of `default(DateTime)`.

This commit fixes that error by adding the proper handling for null
metadata constants with `DateTime` parameters.

src/mscorlib/src/System/Reflection/MdConstant.cs

@@ -94,6 +94,9 @@ public static unsafe Object GetValue(MetadataImport scope, int token, RuntimeTyp
                         defaultValue = buffer;
                         break;
 
+                    case CorElementType.Class:
+                        return null;
+
                     default:
                         throw new FormatException(SR.Arg_BadLiteralFormat);
                         #endregion