is_chain_list: return FALSE for empty lists

[utkarshpawade]

Fixes #516

Summary

• Replaces 9 uses of sapply() with vapply() across 5 files
• vapply() enforces a declared return type, preventing silent list() returns on zero-length input that can cause hard to diagnose downstream failures
• Two sapply(x, colnames) calls at helpers-mcmc.R:277,303 are intentionally unchanged, they rely on flexible return types to detect mismatched column names across chains

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (5c1d015) to head (2ad7ef4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #517   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          35       35           
  Lines        5903     5905    +2     
=======================================
+ Hits         5825     5827    +2     
  Misses         78       78           

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[jgabry]

Thanks. I don't actually think we need these replacements because most of the zero-length cases cannot possibly reach them. The one case I think is worth fixing is just erroring when is_chain_list is passed an empty list, but we can do that immediately before reaching sapply and without needing vapply (see review comment). The switch to vapply doesn't actually solve the problem in that case. So I would just make this PR into one fixing that one specific case.

[jgabry]

I think I accidentally reverted your changes, so I just pushed again to restore your changes. I think it's ready now