Fix IIIF plugin missing last page + erroring when metadata unspecified

[glorieux-f]

Closes #1348 Closes #1350

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.25%. Comparing base (44da86f) to head (f4ef73f). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1349 +/- ## ======================================= Coverage 69.25% 69.25% ======================================= Files 59 59 Lines 5090 5090 Branches 1072 1072 ======================================= Hits 3525 3525 Misses 1538 1538 Partials 27 27 ```

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[cdrini]

Tested:

• The even pages example now works: https://deploy-preview-1349--lucid-poitras-9a1249.netlify.app/bookreaderdemo/demo-iiif?manifest=%20https%3A%2F%2Fglorieux-f.github.io%2Fiiif_tests%2Fbookreader_issue_even-pages.json#mode/2up
• The example without metadata now works: https://deploy-preview-1349--lucid-poitras-9a1249.netlify.app/bookreaderdemo/demo-iiif?manifest=%20https%3A%2F%2Fiiif.io%2Fapi%2Fcookbook%2Frecipe%2F0009-book-1%2Fmanifest.json#mode/2up

[cdrini]

Note the end-to-end testing CI is failing since archive.org is down right now :/ So merging without it.