wfe: Handle empty JSON to /acct like POST-as-GET

ACME clients can make an empty request to /acme/acct for their reg object. wfe.Account handles a truly empty request correctly, but has routed empty JSON objects to updateAccount, even though an update is not appropriate. This only happened to work because UpdateRegistration's impl takes an entire Registration and returns it even if unchanged.

This preserves ACMEv1 backwards compatibility when UpdateRegistration is simplified.

wfe2/wfe.go

@@ -1416,8 +1416,10 @@ func (wfe *WebFrontEndImpl) Account(
 		return
 	}
 
-	// If the body was not empty, then this is an account update request.
-	if string(body) != "" {
+	// If the body was not either completely empty or an empty JSON object, then
+	// this is an account update request. Treating the empty JSON object like a
+	// POST-as-GET is a holdover from ACMEv1.
+	if string(body) != "" && string(body) != "{}" {
 		currAcct, prob = wfe.updateAccount(ctx, body, currAcct)
 		if prob != nil {
 			wfe.sendError(response, logEvent, prob, nil)