clear method Null safety

Implementation

void clear() {
  var exception = _getException();

  final clearConstructor = _dll_handle.lookupFunction<_DynamicTemplateIndex_clear_c, _DynamicTemplateIndex_clear_dart>(_c_namespace + "DynamicTemplateIndex_clear");

  clearConstructor(_impl, exception);

  checkException(exception, _dll_handle);
}