private static String doRenderValue(ExpressionInfo expr) {
    for (ExpressionComparisonRenderer renderer : RENDERERS) {
      String result = renderer.render(expr);
      if (result != null) return result;
    }
    return renderToStringOrDump(expr);
  }
Returns a string representation of a value, or <tt>null</tt> if
the value should not be shown (because it does not add any valuable
information). Note that the method may also change rendered values
of child expression.

@param expr the expression whose value is to be rendered
@return a string representation of the value
spock-core/src/main/java/org/spockframework/runtime/ExpressionInfoValueRenderer.java