Really skip the body while logging in JSON

Co-authored-by: Jonathan Perret <j-github@jonathanperret.net>

app/server/lib/FlexServer.ts

@@ -426,7 +426,7 @@ export class FlexServer implements GristServer {
         host: tokens.host(req, res),
         method: tokens.method(req, res),
         path: tokens.gristInfo(req, res),
-        body: shouldLogBody ? tokens.body(req, res) : {},
+        ...(shouldLogBody ? { body: tokens.body(req, res) } : {}),
         status: tokens.status(req, res),
         timeMs: parseFloat(tokens['response-time'](req, res)) || undefined,
         contentLength: parseInt(tokens.res(req, res, 'content-length'), 10) || undefined,