Handle the case of pylint returning empty output

[hithwen]

Handles the case in which pylint is returning no output in stdout:

 $ pylint -f json --py3k -d W1618 --persistent no --exit-zero /Users/julia.simon/src/integrations-core/apache/datadog_checks/apache
No config file found, using default configuration

$pylint --version
No config file found, using default configuration
pylint 1.9.4,
astroid 1.6.6
Python 2.7.16 (default, Mar  4 2019, 09:01:38)
[GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.11.45.5)]

>>> json.loads('')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/__init__.py", line 348, in loads
    return _default_decoder.decode(s)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

New output

 $ ddev validate py3 apache
Validating python3 compatibility of apache...
	Running: pylint -f json --py3k -d W1618 --persistent no --exit-zero /Users/julia.simon/src/integrations-core/apache/datadog_checks/apache
No config file found, using default configuration
apache is compatible with python3

[codecov]

Codecov Report

Merging #4801 into master will decrease coverage by 16.61%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
riakcs/datadog_checks/riakcs/riakcs.py
oracle/tests/test_oracle.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
...hecks/openstack_controller/openstack_controller.py
...ent_log/datadog_checks/win32_event_log/__init__.py
btrfs/tests/test_btrfs.py
...nmetrics/datadog_checks/openmetrics/openmetrics.py
... and 825 more