Branches page crashes if dataStateAt is empty in branches response

Steps to Reproduce:

1. Remove custom dataStateAt property from snapshot:
   zfs set dblab:datastateat="-" pool@snapshot_20250723091500
2. Open branches page.

Error:

RangeError: Invalid time value

Example response (GET /branches):

[
  {
    "name": "main",
    "parent": "-",
    "dataStateAt": "",
    "snapshotID": "pool@snapshot_20250723091500",
    "dataset": "pool",
    "numSnapshots": 1
  }
]