Be more strict - return None so problems like #348 will trip up the later None check.

Author: ionelmc

src/pytest_cov/engine.py

@@ -101,7 +101,7 @@ def sep(stream, s, txt):
 
     def summary(self, stream):
         """Produce coverage reports."""
-        total = 0
+        total = None
 
         if not self.cov_report:
             with _backup(self.cov, "config"):