Test + support custom constructor edge case

[brentyi]

Thought of another edge case when we simultaneously mix:

• Custom constructors
• Sequence types
• Zero-length inputs
• Positional arguments

Added tests + simple fix.

relevant: #257 / #258

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.86%. Comparing base (fd76d72) to head (9a434d4).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #259   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files          28       28           
  Lines        2250     2250           
=======================================
  Hits         2247     2247           
  Misses          3        3           

Flag	Coverage Δ
unittests	99.86% <ø> (ø)

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