Merge pull request #16640 from mehrdada/fix-ssl-credentials-null

Fix SSL channel credential when an argument is None

src/python/grpcio/grpc/_cython/_cygrpc/credentials.pyx.pxi

@@ -144,8 +144,14 @@ cdef class SSLChannelCredentials(ChannelCredentials):
       return grpc_ssl_credentials_create(
           c_pem_root_certificates, NULL, NULL, NULL)
     else:
-      c_pem_key_certificate_pair.private_key = self._private_key
+      if self._private_key:
-      c_pem_key_certificate_pair.certificate_chain = self._certificate_chain
+        c_pem_key_certificate_pair.private_key = self._private_key
+      else:
+        c_pem_key_certificate_pair.private_key = NULL
+      if self._certificate_chain:
+        c_pem_key_certificate_pair.certificate_chain = self._certificate_chain
+      else:
+        c_pem_key_certificate_pair.certificate_chain = NULL
       return grpc_ssl_credentials_create(
           c_pem_root_certificates, &c_pem_key_certificate_pair, NULL, NULL)