Sometimes being optimistic is useful in particular situations, to help people.
Happy mind tends to be calm, with less vexations; and that helps to put efforts in helping sentient beings, instead of wasting our energy in self-centered vexations.
So it's not advisable to be over-excited with anything, or to attach to anything (because ultimately you can't keep what you are attached to, and so attachments lead to wasting our energy in suffering).
Also, attachments impair the scope of our view.
But if we practice optimism as calm, non-exciting way of living happily, then it helps to keep our perceptions open, unconstrained, and to understand things deeper.
That's why, for example, the practice of dana (giving) helps to develop prajna (wisdom): calm and happy mind is clear and sees well.
As for "not experiencing emotions in nirvana", that sentence can be easily misunderstood.
I would advise you to investigate where it comes from and what it actually means.
There are mistranslations and misconceptions about that even in books of some famous Buddhist teachers.
For example, Khyentse says that "All emotions are painful". However, that's true only for a very specific understanding of the term "emotions", not quite corresponding with what we consider to be emotions in our daily experience.
We can understand "emotions" as just feelings that carry useful signals. It's a part of our mental abilities, and there is nothing wrong with them.
Only when they become attached to, problems start to arise.
In nirvana — phenomena happen, including emotional phenomena; trying to "get rid of emotions" might be a wrong kind of practice.