volume snapshots can have an empty expiry (#1011)

## Done

- fix volume snapshots, they can have an empty expiry, and we should
show that correctly

## QA

1. Run the LXD-UI:
- On the demo server via the link posted by @webteam-app below. This is
only available for PRs created by collaborators of the repo. Ask
@mas-who or @edlerd for access.
- With a local copy of this branch, [build and run as described in the
docs](../CONTRIBUTING.md#setting-up-for-development).
2. Perform the following QA steps:
- create a custom volume, create snapshots without a name or expiry on
that volume, browse the snapshot list of that volume.

## Screenshots

Before:


![image](https://github.com/user-attachments/assets/acf34012-3698-4f79-a091-e1c10e678e51)


After:


![image](https://github.com/user-attachments/assets/535109f4-51c6-49c7-9864-5992056d2fb7)

src/util/helpers.tsx

@@ -13,7 +13,7 @@ import { FormDevice } from "./formDevices";
 export const UNDEFINED_DATE = "0001-01-01T00:00:00Z";
 
 export const isoTimeToString = (isoTime: string): string => {
-  if (isoTime === UNDEFINED_DATE) {
+  if (isoTime === UNDEFINED_DATE || !isoTime) {
     return "";
   }