docs: @SuppressWarnings({"unused", "WeakerAccess"})
      public void addMultiValueForKey(String key, String value) {
  
          //noinspection ConstantConditions
          if (value == null || value.isEmpty()) {
              _generateEmptyMultiValueError(key);
              return;
          }
  
          addMultiValuesForKey(key, new ArrayList<>(Collections.singletonList(value)));
      }