Fixed decoding warning for None tags

[Simwar]

What does this PR do?

If an integration tries to send a metric with tag set to None, there will be a warning thrown by this code: https://github.com/DataDog/integrations-core/blob/master/datadog_checks_base/datadog_checks/base/checks/base.py#L352-L363
As we cannot decode it.

Now, we simply ignore if a tag is set to None.

Motivation

Customer which had his logs spammed by this warning, because of a label missing in a prometheus payload.

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[masci]

since we expect None tags, let's explicitly do if tag is None: continue and leave the rest of the code unchanged

[Simwar]

✅

[masci]

let's test the contents, not the length

[Simwar]

✅ thanks!

[codecov]

Codecov Report

Merging #3249 into master will decrease coverage by 9.2%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3249      +/-   ##
==========================================
- Coverage   85.18%   75.98%   -9.21%     
==========================================
  Files         690       54     -636     
  Lines       36813     3964   -32849     
  Branches     4469      504    -3965     
==========================================
- Hits        31360     3012   -28348     
+ Misses       4204      832    -3372     
+ Partials     1249      120    -1129

[Simwar]

Added a follow-up PR to have the same fix but for the python2 check base class: #3665