Fix TypeError in schema size check

[rayatbuzzfeed]

What does this PR do?

This fixes a TypeError exception when it returns null from a non base table such as those in the view or sys db

TypeError: long() argument must be a string or a number, not 'NoneType'\
\"}]"}

Motivation

Getting pass the above error in checks. It now returns 0 if null instead of none.

example DBs that would return null:

mysql> select table_schema, data_length, index_length from information_schema.tables group by table_schema;
+--------------------+-------------+--------------+
| table_schema       | data_length | index_length |
+--------------------+-------------+--------------+
| information_schema |           0 |            0 |
| mysql              |      515970 |        53248 |
| performance_schema |           0 |            0 |
| sys                |        NULL |         NULL |
| tmp                |       16384 |        32768 |
| view               |        NULL |         NULL |
+--------------------+-------------+--------------+
9 rows in set (0.04 sec)

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• 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

[codecov]

Codecov Report

Merging #5043 into master will decrease coverage by 9.86%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
mysql/datadog_checks/mysql/mysql.py	71.71% <ø> (ø)
...checks_dev/tests/plugin/test_environment_runner.py
riakcs/datadog_checks/riakcs/riakcs.py
oracle/tests/test_oracle.py
...r/datadog_checks/openstack_controller/__about__.py
twistlock/datadog_checks/twistlock/twistlock.py
linkerd/datadog_checks/linkerd/linkerd.py
...oller/datadog_checks/openstack_controller/utils.py
istio/tests/test_e2e.py
disk/datadog_checks/disk/__init__.py
... and 844 more