Delete empty nodes instead of merging into them

When you press delete inside an empty node (e.g. Heading) that
node should be removed, instead of the paragraph beneath it being
merged into and effectively converted. If the heading is non-empty
then merging is still the correct behaviour.

Also add in test case.

Bug: 50254
Change-Id: If9cee79feb4b4ee9d7c367e392b00fee5e8c0669