vm/qubesvm: fix handling empty kernel value

Do not fail on empty kernel value - which means "use booloader on
root.img".

Fixes 3ddc052 "vm: move kernel presence validation to event handler"

qubes/tests/vm/qubesvm.py

@@ -360,6 +360,11 @@ class TC_90_QubesVM(QubesVMTestsMixin, qubes.tests.QubesTestCase):
         self.assertPropertyInvalidValue(vm, 'kernel', 123)
         self.assertPropertyInvalidValue(vm, 'kernel', 'invalid')
 
+    def test_252_kernel_empty(self):
+        vm = self.get_vm()
+        self.assertPropertyValue(vm, 'kernel', '', '', '')
+        self.assertPropertyValue(vm, 'kernel', None, '', '')
+
     def test_260_kernelopts(self):
         vm = self.get_vm()
         self.assertPropertyDefaultValue(vm, 'kernelopts',

qubes/vm/qubesvm.py

@@ -755,6 +755,8 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
     @qubes.events.handler('property-pre-set:kernel')
     def on_property_pre_set_kernel(self, event, name, newvalue, oldvalue=None):
         # pylint: disable=unused-argument
+        if not newvalue:
+            return
         dirname = os.path.join(
             qubes.config.system_path['qubes_base_dir'],
             qubes.config.system_path['qubes_kernels_base_dir'],