Allow for optional prompting

[brayevalerien]

Since the first stage sampler allows for passing positive and negative prompts, why not allow the user to write these prompts instead of leaving them blank (which probably induces a bias in fact)?

I don't know if it would have a lot of influence on the result but it could probably slightly help handling different textures better, for instance by prompting "skin texture" for portraits and "tree, leaves" for forest landscapes.

However I do understand that adding another two parameters that probably are not that useful could lead to a heavier node so you might have done that by design.