Allow empty JSON in exec plugin

[danielnelson]

Feature Request

As suggested by @mjesse88, when using json data_format the output should be optional, similar to how the influx data_format works.

Proposal:

Current behavior:

test.bat

@echo off
exit /b 0
E! Error in plugin [inputs.exec]: unable to parse out as JSON, unexpected end of JSON input

2.test.bat

@echo off
echo ""
exit /b 0
E! Error in plugin [inputs.exec]: unable to parse out as JSON, json: cannot unmarshal string into Go value of type map[string]interface {}

Desired behavior:

No error or metric produced.

Use case: [Why is this important (helps with prioritizing requests)]

Sometime I don't need metrics write to influxdb, so my script output empty json,

[mjesse88]

Thanks for your attention

Jesse