Guard against manifests without sequences

[cbeer]

Fixes #2940

TODO:

• [ ] collect some examples of manifests without sequences
• [ ] add some integration tests for them

[codecov-io]

Codecov Report

Merging #2939 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2939      +/-   ##
==========================================
+ Coverage   93.23%   93.23%   +<.01%     
==========================================
  Files         158      158              
  Lines        2351     2352       +1     
==========================================
+ Hits         2192     2193       +1     
  Misses        159      159

Impacted Files	Coverage Δ
src/state/selectors/canvases.js	97.53% <100%> (ø)	:arrow_up:
src/state/selectors/manifests.js	98.14% <100%> (ø)	:arrow_up:
src/components/ViewerInfo.js	100% <100%> (ø)	:arrow_up:

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