Fix Filter::getTimePeriodValues() function (skip null values)

[pastakhov]

• It returns "0-9" in possibleFilterValues when the first value is null.
• Also I don't think there is sense to call $timePeriod = $this->timePeriod() every loop.

[codecov-commenter]

Codecov Report

Merging #49 (b1922a1) into master (c517061) will decrease coverage by 0.02%.
The diff coverage is 75.00%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@             Coverage Diff              @@
##             master      #49      +/-   ##
============================================
- Coverage     68.98%   68.96%   -0.02%     
- Complexity      740      741       +1     
============================================
  Files            36       36              
  Lines          2086     2088       +2     
============================================
+ Hits           1439     1440       +1     
- Misses          647      648       +1     

Impacted Files	Coverage Δ
includes/Filter.php	81.00% <75.00%> (-0.32%)	⬇️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[gesinn-it-gea]

@pastakhov please always create an issue before a PR so that it's easier to reference or discuss changes beforehand.

[gesinn-it-wam]

Did you consider writing a failing test before fixing the issue? (Something along the lines of filters.json in a separate test file date-filters.json) Having a problematic setting would make code review much easier! (Not to mention the many other benefits of having tests...)