[BUG] Get snapshot API returns incorrect response

[gaobinlong]

Describe the bug
When calling Get snapshot API with setting ignore_unavailable parameter to true, if the snapshot does not exist, then this API will return all of the executing snapshots which are in IN_PROGRESS state.

To Reproduce

1. Create a snapshot, which will take a long time to execute.
2. Call the Get Snapshot API with an non-existing snapshot and setting ignore_unavailable parameter to true:
   GET _snapshot/test/xx?ignore_unavailable=true
   then you can see that the response contains the snapshot which is executing and in IN_PROGRESS state.

Expected behavior
Return empty response if the snapshot does not exist.

Screenshots

Host/Environment (please complete the following information):

• OS: [Centos]
• Version [2.4.1]