Json formatter bypasses redirect_stdout

[Martmists-GH]

Steps to reproduce

1. Install pylint
2. Run it like this:

from contextlib import redirect_stdout
from io import StringIO
from pylint.lint import Run

file = StringIO()
with redirect_stdout(file):
    Run(["some_file.py"], do_exit=False)  # No output


file_2 = StringIO()
with redirect_stdout(file_2):
    Run(["-f", "json", "some_file.py"], do_exit=False)  # JSON output

Current behavior

There is still being written to stdout (I checked stderr with the same method, but according to the source stdout is being used)

Expected behavior

redirect_stdout catches all content written to stdout and writes it to the StringIO instead

pylint --version output

$ pylint --version
pylint 2.4.3
astroid 2.3.1
Python 3.7.4 (default, Oct  4 2019, 06:57:26) 
[GCC 9.2.0]

Cause

json_reporter.py:25
JSONReporter has sys.stdout as default parameter, which causes it to keep a reference to the original output.
To solve this, simply replace sys.stdout with None as it will fetch sys.stdout if the output parameter is falsy.