Fix srcset lookup

[Lukewh]

Handle images within a discourse topic - https://discourse.charmhub.io/t/endpoint/5462 for example - that don't have srcset.

QA:

• Pull this branch
• Clone juju.is
• Update juju.is requirements.txt:

  #canonicalwebteam.discourse==5.4.4
  -e ./canonicalwebteam.discourse

• Within juju.is run dotrun -m "/path/to/canonicalwebteam.discourse":"canonicalwebteam.discourse" (you need the most up to date version of dotrun)
• Visit http://localhost:8041/docs/juju/endpoint and ensure the images at the bottom of the page load