Add unit tests for sonic_data_client/db_client.go

[yxieca]

Problem\nThe module appears to lack direct unit tests. There are unit tests elsewhere in the repo, but no -specific tests (e.g., no ). This makes it easier for regressions like empty-slice access in to slip through.\n\n### Proposal\nAdd unit tests covering and paths, including cases where resolves to an empty slice and non-empty slices with/without .\n\n### Benefit\nImproves regression protection for gNMI subscription handling in and prevents panics on edge cases.