better NA handling in data frames

[schochastics]

Description

A PR in igraph igraph/rigraph#1828 introduced an error if graphs with NA vertices/edges are constructed. This causes issues with manynet. See
https://github.com/igraph/rigraph/blob/main/revdep/problems.md#manynet

This PR fixes the issue by handling NAs before the graph creation.

Checklist:

• PR form
  • Description above itemizes changes under subtitles, e.g. "## Data""
  • Any closed, fixed, or related issues are referenced and explained in the description above, e.g. "Fixed #0 by adding A"
  • Package builds on my OS without issues
• PR checks all pass for latest commit
  • CodeFactor check: Package improves or maintains good style
  • Package builds on Mac
  • Package builds on Windows
  • Package builds on Linux
  • CodeCov check: Package improves or maintains good test coverage
• Documentation
  • Any new or modified functions or data have roxygen style documentation in their .R scripts
  • Longer functions are commented inline or broken down into helper functions so that it is easier to debug in the future
  • PR description above and the NEWS.md file are aligned
  • DESCRIPTION file version is bumped by the appropriate increment (major, minor, patch)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.43%. Comparing base (17b07f4) to head (ef37bd2).
Report is 808 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #109      +/-   ##
===========================================
- Coverage    47.94%   43.43%   -4.52%     
===========================================
  Files           27       51      +24     
  Lines         3604     6923    +3319     
===========================================
+ Hits          1728     3007    +1279     
- Misses        1876     3916    +2040     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jhollway]

Great, thanks @schochastics