VaultPkiOperations.getCrl(…) throws a NullPointerException when CRL is absent

[njambe]

On VaultPkiTemplate, the getCrl method throws a NPE when no certificate is revoked on the PKI.

The REST call '{path}/crl' (also valid for PEM call) returns a 204 No Content with a null body. The response should be checked against a null body before trying to initialize the ByteArrayInputStream.

Tested with Vault 1.3.2