Annotations received from the API always have a non-empty `target` array, as do
annotations and replies created locally that have not yet been saved. Unsaved
Page Notes were an exception. This led to an error [1] when hovering a new
unsaved page note in a VitalSource book, due to code not handling an empty
`target` array.  Resolve this by initializing the `target` field of new page
notes in the same way as replies and annotations (minus the `selector` property
that annotations have).

[1] https://hypothesis.sentry.io/issues/3745569320/