[KeyVault] Addressed feedback on samples

[sadasant]

Jonathan found that some of the samples have issues. Here's a PR fixing them.

Since this is only in the samples, perhaps this doesn't affect the current release build?

Fixes #5444

[sophiajt]

This looks like it still gives the error: "Error: The 'version' cannot be empty."

[sophiajt]

Feels like we should land these, though I noticed a couple related issues that aren't addressed quite yet. I left a comment about one.

backupAndRestore for certificates also gave me this error:

error code:  Conflict
error message:  Conflict while restoring certificate https://joturner-keyvault4.vault.azure.net/certificates/MyCertificate2336/322018317dd541f793d6fbf3a6d0f15d - certificate already exists or concurrent access
error stack:  Error: Conflict while restoring certificate https://joturner-keyvault4.vault.azure.net/certificates/MyCertificate2336/322018317dd541f793d6fbf3a6d0f15d - certificate already exists or concurrent access
    at new RestError (C:\Source\preview4\certs\node_modules\@azure\core-http\dist\coreHttp.node.js:1698:28)
    at C:\Source\preview4\certs\node_modules\@azure\core-http\dist\coreHttp.node.js:2915:37
    at processTicksAndRejections (internal/process/task_queues.js:93:5)

In my version I changed it to:

  await client.deleteCertificate("MyCertificate2338");

  // It might take less time, or more, depending on your location, internet speed and other factors.
  await delay(30000);

  await client.purgeDeletedCertificate("MyCertificate2338");
  await delay(30000);

And that seemed to work