Account for null client certificate

If a null certificate is passed to setClientKeyEntry() we should
not pass a non-null array with a single null element to the
setClientKeyEntryWithCertificateChain helper method.  Instead we
should pass a null array.

Cherry-pick of 410a3498

Bug: 34765004
Test: cts-tradefed run cts -d --module CtsNetTestCases --test android.net.wifi.cts.WifiEnterpriseConfigTest
Change-Id: I02793b4b29bc7325f98833c58bf652ba68353827