Fix bug in v1.14.4 that broke all GraphQL requests

We had been implicitly assuming the default context was an empty object.
After switching to graphql-http, it's now undefined, so we have to set
that object explicitly instead.

Author: pimterry

src/api/graphql-api.ts

@@ -170,5 +170,8 @@ export function exposeGraphQLAPI(
         resolvers: buildResolvers(apiModel)
     });
 
-    server.post('/', createGraphQLHandler({ schema }));
+    server.post('/', createGraphQLHandler({
+        schema,
+        context: () => ({}) // Fresh empty context for every request
+    }));
 }