docs: protected final String resolvePlaceholder(final String placeholderWithDefault, final Properties props,
                                                final int systemPropertiesMode) {
          String placeholder = getPlaceholder(placeholderWithDefault);
          String resolved = super.resolvePlaceholder(placeholder, props, systemPropertiesMode);
          if (resolved == null) {
              resolved = getDefault(placeholderWithDefault);
          }
          return resolved;
      }