Fix Annotation.__init__ to handle length-zero Sequence, fixes #290

[NickleDave]

Fix for #290 -- changes Annotation.__init__ so we check if seq is not None: instead of just doing if seq:, to avoid the edge case where a length-zero Sequence with no segments evaluates to False. Adds a unit test to check for this

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.08%. Comparing base (97e0dd0) to head (c683d29).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #291      +/-   ##
==========================================
- Coverage   94.20%   94.08%   -0.12%     
==========================================
  Files          43       43              
  Lines        1759     1692      -67     
==========================================
- Hits         1657     1592      -65     
+ Misses        102      100       -2     

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