fix: checking nil values when reading secret in vault provider

pkg/provider/vault/vault.go

@@ -219,6 +219,9 @@ func (v *client) readSecret(ctx context.Context, path, version string) (map[stri
 
 	byteMap := make(map[string][]byte, len(secretData))
 	for k, v := range secretData {
+		if v == nil {
+			continue
+		}
 		switch t := v.(type) {
 		case string:
 			byteMap[k] = []byte(t)