checksums: Never skip checksumming phase

It does not make sense for this phase to be skipped. If there are any
images, we need to generate the checksums so that writing metadata
works. If there are no images, the phase does not do anything and is
therefore very fast.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_imagechecksumphase.py

@@ -20,6 +20,11 @@ from tests.helpers import DummyCompose, PungiTestCase
 
 class TestImageChecksumPhase(PungiTestCase):
 
+    def test_phase_is_never_skipped(self):
+        compose = DummyCompose(self.topdir, {})
+        phase = ImageChecksumPhase(compose)
+        self.assertFalse(phase.skip())
+
     def test_config_skip_individual_with_multiple_algorithms(self):
         compose = DummyCompose(self.topdir, {
             'media_checksums': ['md5', 'sha1'],