Fix #2317 empty pages CEUR-WS scraper

[fnielsen]

CEUR-WS scraper failed where metadata had a element for pages but this was empty.

Fixes #2317

Description

Test on empty string for empty pages in pages element in scraped page for CEUR-WS

Caveats

Please list anything which has been left out of this PR or which should be considered before this PR is accepted
Check any of the following which apply:

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
  • I have made corresponding changes to the documentation
• This change requires new dependencies (please list)

If you make changes to the Python code

• My code passes the tox check, I can receive warnings about tests, documentation or both

Testing

Please describe the tests that you ran to verify your changes. Provide instructions, so we can reproduce. Please also list any relevant details for your test configuration.

• python -m scholia.scrape.ceurws proceedings-url-to-quickstatements https://ceur-ws.org/Vol-3443/

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have not used code from external sources without attribution
• I have considered accessibility in my implementation
• There are no remaining debug statements (print, console.log, ...)