Don't create empty inserts in delta::Builder
Don't create empty inserts in delta::Builder

The multiset::SubsetBuilder segments require that all segments pushed are non-
empty. This caused a crash when typing ctrl+y (yank) with nothing in the kill
ring would use delta::Builder::replace to insert an empty string, which would
end up constructing an empty segment. I fixed this by not allowing
delta::Builder to construct empty Insert nodes.

modelorganism/xi-editor,google/xi-editor,google/xi-editor,google/xi-editor,modelorganism/xi-editor,modelorganism/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor