fix VDH's (un-)handling of incomplete video container metadata

[keighrim]

fixes#390

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.81%. Comparing base (21ca1e0) to head (3428ebf).

Files with missing lines	Patch %	Lines
mmif/utils/video_document_helper.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## develop #391 +/- ##
===========================================
+ Coverage 74.78% 74.81% +0.03% 
===========================================
Files 27 27 Lines 3228 3240 +12 ===========================================
+ Hits 2414 2424 +10 - Misses 814 816 +2 

Flag	Coverage Δ
unittests	74.81% <95.83%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 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.