code: public static <T> ExpressionList<T> eqIfNotNull(ExpressionList<T> expressionList,
                                                      String propertyName,
                                                      Object value) {
          Assert.notNull(expressionList, "expressionList must not null");
          Assert.hasText(propertyName, "propertyName must not null");
          if (value != null) {
              return expressionList.eq(propertyName, value);
          }
          return expressionList;
      }
docs: Return a ExpressionList specifying propertyName equals value.
  
  @param expressionList the ExpressionList to add contains expression
  @param propertyName   the property name of entity bean.
  @param value          equals value
  @param <T>            the type of entity.
  @return a ExpressionList specifying propertyName equals value.