Fix Save Figure with no panels

[will-moore]

Fixes #336

To test:

• Create a new figure (e.g. with a single image)
• Delete all panels so you have a blank page
• Save -> Choose a name -> OK
• Figure should get created. Can see it in File > Open etc.
• Can Open the file and add images later etc....

[joshmoore]

The change itself looks simple enough; 👍 for merging.

I'd like to point out though that there's some overlap in the if-statements just above and below e.g. a check for if first_img_id is not None: on 249. Glancing over more of the code, I then saw an "upload file" duplicated on line 286. Similar to @jburel's recent ome/training-scripts#80, I think we will need to start taking the opportunities of these simple fixes to also reduce overall code complexity. This change doesn't make it much harder to read, but in the sum, someone would need to think a bit longer the next time they come back to this section.

[pwalczysko]

OMERO.figure does not seem to work on merge-ci

[will-moore]

Figure on merge-ci was temporarily broken by 'Figure CLI' PR. Working now.

[pwalczysko]

Works as expected on merge-ci. Ready to merge fmpov