code: protected final String setRequiredValueAttribute(String attrValue, String attrName)
              throws JspException
      {
          assert(attrValue != null) : "parameter '" + attrValue + "' must not be null";
          assert(attrName != null) : "parameter '" + attrName + "' must not be null";
  
          if ("".equals(attrValue)) {
              String s = Bundle.getString("Tags_AttrValueRequired", new Object[]{attrName});
              registerTagError(s, null);
              return null;
          }
          return attrValue;
      }
docs: Report an error if the value of <code>attrValue</code> is equal to the empty string, otherwise return
  that value.  If <code>attrValue</code> is equal to the empty string, an error is registered and
  null is returned.
  @param attrValue The value to be checked for the empty string
  @param attrName  The name of the attribute
  @return either the attrValue if it is not the empty string or null
  @throws JspException A JspException will be thrown if inline error reporting is turned off.