autopkgtest.py: return an empty list instead of tests

While tests is an empty list at this point we should make it more
obvious by returing an empty list which is what the rest of the code
does.

britney2/policies/autopkgtest.py

@@ -712,7 +712,7 @@ class AutopkgtestPolicy(BasePolicy):
                     self.logger.info('Source package %s has binaries which are all Architecture: all, and tests have been requested on %s, not running any tests for this src package',
                                      src,
                                      arch)
-                    return tests
+                    return []
         except Exception as e:
             self.logger.error('i386 useless autopkgtest check failed with: %s', e)