Cert path: sigProvider is not required

Some calls to X509Certificate#verify in the CertPathVerifier and
CertPathBuilder code were passing in sigProvider even when it was null
which causes the GetInstance code to throw an error since this is an
invalid condition.

Bug: 26954162
Change-Id: Id8afa2c786a0c6d44127857950d5f3bae60da11a