Enable wolfSSL_i2d_X509_NAME_canon to handle blank optional fields

[embhorn]

Description

Currently, null item in the certificate name is considered an error, but having a blank optional field should be OK.

Fixes zd20913

Testing

Customer confirmed

Checklist

• [ ] added tests
• [ ] updated/added doxygen
• [ ] updated appropriate READMEs
• [ ] Updated manual and documentation