html_node_to_text doesn't emit src-less images

fixes CNVS-13449

test plan
- create a discussion topic with an image without a src attribute
- ensure that notification is sent for the topic

Change-Id: Idb87eaa955089197aaaa2247cc6172d8dd52c6ba
Reviewed-on: https://gerrit.instructure.com/36756
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <braden@instructure.com>
QA-Review: David Josse <david@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>