webadmin: A NPE check in ImportVmModel:init method.

There's a possible branch of statement, that if executed
a null value will be dereferenced. this patch verifies that the var
dataCenter is not null before using it.

Change-Id: Icdba43d13299d0768c1449c450de61cb6b0a3df9
Signed-off-by: Asaf Shakarchi <asaf@redhat.com>