When a RSA key is generated (generateRsaKeyAndSelfSignedCertificate) this passphrase is saved in flash and when the time comes and the certificate and the key are exported in PKCS12 format this passphrase is used to encrypt it. If the passphrase is empty the key and certificate can not be exported. There is no method of obtaining this passphrase once a key was generated.
Sources for this description
Multiple manufacturers; see the original MIB MIB copies (LibreNMS/Observium Community)Dell MIB copies (LibreNMS/Observium Community)Netdisco MIB archive copiesNETGEAR official product and EOL MIB downloads