Coverity static analysis: Explicit null dereferenced in FileSystemSecurityRealm (Elytron)

Coverity static-analysis scan found possible use of null object in FileSystemSecurityRealm.parseCredential() method.

https://scan7.coverity.com/reports.htm#v16159/p11778/fileInstanceId=5760040&defectInstanceId=1541373&mergedDefectId=1369298&eventId=1541373-14
If the format is not provided as an attribute in the provided XML stream, then the call

  String format = null;
  // ...
  function.parseCredential(algorithm, format, text);

will fail for parsePublicKey method as the function code it contains code calling method of the format parameter (with possible null value).

if (! format.equals("pkcs8")) {
    throw ElytronMessages.log.fileSystemRealmUnsupportedKeyFormat(format, path, streamReader.getLocation().getLineNumber(), name);
}