Fix KVV2 datasource upon retrieval of soft deleted secrets

[vinay-gopalan]

This PR adds a type cast check for the data returned on reading a KV V2 secret from Vault. The current implementation panics when trying to retrieve a secret that has been soft deleted (the secret still exists, but the data attribute is nil). The PR also adds a test for this use-case and confirms that the fix works as expected.

Fixes #1731

$ make testacc TESTARGS='-v -run TestDataSourceKVV2Secret_deletedSecret'
=== RUN   TestDataSourceKVV2Secret_deletedSecret
--- PASS: TestDataSourceKVV2Secret_deletedSecret (0.98s)
PASS