Avoid TypeError when running at PeriodicItemCountMonitor

[VMRuiz]

Fixes https://github.com/scrapinghub/spidermon/issues/427

However, the monitor will still won't pass until item_scraped_count is defined. I'm not sure if we should include the possibility of wanting this monitor to pass when tresshold is 0 and item_scraped_count is None.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.43%. Comparing base (8493465) to head (98075ce).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #436 +/- ## ======================================= Coverage 79.43% 79.43% ======================================= Files 76 76 Lines 3229 3229 Branches 536 536 ======================================= Hits 2565 2565 Misses 593 593 Partials 71 71 ```

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