Ensure metadata.display_defaults exists

A bug was introduced in PR #1280 where datasets which did not
define `metadata.display_defaults` would crash, as the code assumed
its existence. This property is optional in the dataset
JSON.

This commit ensures `display_defaults` exists in redux state after
a dataset is loaded, thus allowing code to rely on its presence.
This was preferred to checking for `display_defaults` in (all) the
relevant sections of code, now and in future.

(Using TypeScript, or expanding our smoke tests, would be approaches
to avoiding these kinds of bugs in future.)

CHANGELOG.md

@@ -2,9 +2,9 @@
 title: Changelog
 ---
 
-## version 2.24.0 - 2021/03/17
-
+* [bugfix] Fixes a bug introduced in v2.24.0 where certain datasets wouldn't load
 
+## version 2.24.0 - 2021/03/17
 
 * Frequencies are no longer normalized when the data is lacking.
 See [PR 1278](https://github.com/nextstrain/auspice/pull/1278) for more.

src/actions/recomputeReduxState.js

@@ -241,6 +241,8 @@ const modifyStateViaMetadata = (state, metadata) => {
         }
       }
     }
+  } else {
+    metadata.displayDefaults = {}; // allows code to rely on `displayDefaults` existing
   }
 
   if (metadata.panels) {