Null check for certificate credential information

Application/x-pkcs12 section with PKCS12 container inside,
the private key and the client certificate of which will
fill out Credential#mClientPrivateKey and
Credential#mClientCertificateChain fields of
PasspointConfiguration#mCredential accordingly. A passpoint profile (xml file)
which has no CertCredentials info (no DigitalCertificate node) will set
Credential#mCertCredential to null.  When user install the certificate
Wi-Fi framework tries to access the contents of Credential#mCertCredential
without a null check which leads to null pointer exception.

So added null check for Credential#mCertCredential to avoid null pointer exception.

Bug: 340853797
Test: Manual - Connect / Disconnect to Passpoint network
Change-Id: I114d5a7e1fbcf319f505e32c5b44133fa480f5b5