fixes #2721, is ad-hoc fix, needs a deeper think to be not-ad-hoc

it was onFocus of a newly created note that triggered an incorrect dispatch of setCursor:

const simplePath = simplifyPath(state, path) || path
.
.
.
setCursor({
  path: simplePath,
  .
  .
}

the problem was: simplifyPath would return an empty array at times like this.
The and we'd get simplifyPath(state, path) || path) = [] || path=[]` , so the fallback never really worked 😄

After putting a more elaborate condition, and using path as a fallback, we get broken editor behavior: the part of the tree collapses(we go to a wrong path)

In the end, when I just did not allow [] paths, it fixed the thing. And the app, from my manual tests, works just fine. The cursor is there, everything is smooth.
setDescendant action puts the appropriate state, from what I observed.

and on the reasons why we even get to the case of simplifyPath returning [] here, I'm not really sure, and feels like, we should dig there, if we want more than just a ad-hoc bug fix.