There are three arguments against your claim that it is more easy to evoke sadness than happiness through any type of text:
(1) There are experiences, such as the loss of an unborn or newborn child, that will universally evoke sadness. The reasons for happiness appear much more varied and individual. But I'm sure you could find things that make most people happy, if you think about it: some reward (just don't specify it), sunshine, the return of a loved person, etc. It is just that out culture is preoccupied with negative emotions (lies, distrust, fear, anxiety etc.), and that therefore we find these emotions much easier to frame with words. After all, a story that plods happily along bores us. It is even claimed that a utopian life without conflict would bore us to tears (which I don't believe, but is commonly accepted as a valid argument against striving for it). And the happy ending of a tale is a sign of unrealistic kitsch, isn't it? Real life is seen to end at best neutrally and usually bad. People who feel good about their lives fill us with the suspicion that something is not quite right with them. Evoking happiness is just as easy, but we find it harder to think of.
(2) Most readers will no actually feel sad after reading Hemingway's sentence, especially not strongly, as you claim, only be reminded of sadness and consider the idea of sadness after the loss of a child cognitively. I'm generally a very empathic person and often cry over nothing, but Hemingway's sentence barely passed the door to the garden with the well of sadness for me.
(3) Also, while sadness is a mild emotion, happiness is an extreme emotion (maximally pleasurable). Evoking happiness is in fact not harder than evoking despair (maximum sadness). So you are in fact comparing apples and oranges here, because I have never read any fictional text that made me feel real despair. There are lots of texts that evoke a pleasant feeling in me, comparable to the vague sadness some others evoke.