autopkgtest: Don't cache results for undefined versions

If a result.tar does not contain a testpkg-version, we must still match it
against pending.txt, but we must not add it to the results cache. This ends up
being a "null" version key (JSON's serialization of None) which becomes an
actual version string once this is read back.

autopkgtest.py

@@ -398,6 +398,7 @@ class AutoPackageTest(object):
 
         # add this result
         src_arch_results = self.test_results.setdefault(src, {}).setdefault(arch, [stamp, {}, False])
+        if ver is not None:
             if passed:
                 # update ever_passed field
                 src_arch_results[2] = True

tests/test_autopkgtest.py

@@ -818,6 +818,21 @@ lightgreen 1 i386 green 3
 
         self.assertEqual(self.pending_requests, '')
 
+        # one more tmpfail result, should not confuse britney with None version
+        self.swift.set_results({'autopkgtest-series': {
+            'series/i386/l/lightgreen/20150101_100201@': (16, None),
+        }})
+        self.do_test(
+            [],
+            NOT_CONSIDERED,
+            [r'\blightgreen\b.*>1</a> to .*>2<',
+             r'autopkgtest for lightgreen 2: .*amd64.*Regression.*i386.*Regression'],
+            ['in progress'])
+        with open(os.path.join(self.data.path, 'data/series-proposed/autopkgtest/results.cache')) as f:
+            contents = f.read()
+        self.assertNotIn('null', contents)
+        self.assertNotIn('None', contents)
+
     def test_rerun_failure(self):
         '''manually re-running failed tests gets picked up'''