webadmin: Fix FindBugs warning in ImportVmModel

In ImportVmModel.init() method, 'dataCenter' reference
variable can theoretically be null, which triggers FindBugs
warning "Possible null pointer dereference".

This patch fixes the above mentioned warning.

Change-Id: Ib0b452b0a998068250dc4431f1f57346f7888000
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>