Be consistent about that finding no key in the provider is a success

All different provider types except Vault treated finding no key as
SUCCESS but with NULL as the key. Let's do this for Vault too which
slightly simplifies the callers which used to have to understand both
ways to handle a key not existing.

contrib/pg_tde/src/catalog/tde_principal_key.c

@@ -256,7 +256,7 @@ set_principal_key_with_keyring(const char *key_name, const char *provider_name,
 
 		keyInfo = KeyringGetKey(new_keyring, key_name, &kr_ret);
 
-		if (kr_ret != KEYRING_CODE_SUCCESS && kr_ret != KEYRING_CODE_RESOURCE_NOT_AVAILABLE)
+		if (kr_ret != KEYRING_CODE_SUCCESS)
 		{
 			ereport(ERROR,
 					errmsg("failed to retrieve principal key from keyring provider :\"%s\"", new_keyring->provider_name),
@@ -333,7 +333,7 @@ xl_tde_perform_rotate_key(XLogPrincipalKeyRotate *xlrec)
 	new_keyring = GetKeyProviderByID(xlrec->keyringId, xlrec->databaseId);
 	keyInfo = KeyringGetKey(new_keyring, xlrec->keyName, &kr_ret);
 
-	if (kr_ret != KEYRING_CODE_SUCCESS && kr_ret != KEYRING_CODE_RESOURCE_NOT_AVAILABLE)
+	if (kr_ret != KEYRING_CODE_SUCCESS)
 	{
 		ereport(ERROR,
 				errmsg("failed to retrieve principal key from keyring provider: \"%s\"", new_keyring->provider_name),

contrib/pg_tde/src/keyring/keyring_vault.c

@@ -236,7 +236,6 @@ get_key_by_name(GenericKeyring *keyring, const char *key_name, KeyringReturnCode
 
 	if (httpCode == 404)
 	{
-		*return_code = KEYRING_CODE_RESOURCE_NOT_AVAILABLE;
 		goto cleanup;
 	}