fix(summary): Verify context.json is an object before looking up keys

[effigies]

When a JSON file is empty (e.g., https://openneuro.org/datasets/ds003404/versions/1.0.0/file-display/sub-C39:fmap:sub-C39_dir-PA_run-1_epi.json), the context.json object does not get populated. Parsing JSON could also theoretically drop in any JSON data structure, so this checks that it is an object before trying to look up TaskName.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.35%. Comparing base (93868ab) to head (f2bab87).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2029 +/- ## ========================================== + Coverage 85.75% 87.35% +1.59% ========================================== Files 91 132 +41 Lines 3785 6327 +2542 Branches 1218 1528 +310 ========================================== + Hits 3246 5527 +2281 - Misses 453 709 +256 - Partials 86 91 +5 ```

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