Prevent cascades and recomputes of removed sections.

Potential fix for underlying issue of #1273. In addition to simple null guard for partials, keypaths beyond end of spliced array are prevented from cascades and re-computations.

@Rich-Harris Doesn't break any tests, but I didn't see lots of dependency scenario tests other than gh issues regressions. Let me know if you can think of cases where this wouldn't make sense.