Ensure FoldMap always contains at least one transform

Previously, when splicing an edit that deleted all transforms, we would
leave the `FoldMap` empty, thus violating a bunch of downstream
invariants and e.g. causing the `BufferRows` iterator to not return any
buffer row.

This commit ensures we always have at least one transform (an isomorphic
one, specifically) and adds additional test coverage for the
`FoldMap::buffer_rows` method by adding it to the randomized tests.

zed/src/sum_tree/mod.rs

@@ -102,7 +102,6 @@ impl<T: Item> SumTree<T> {
         }
     }
 
-    #[cfg(test)]
     pub fn is_empty(&self) -> bool {
         match self.0.as_ref() {
             Node::Internal { .. } => false,