Check "CHART_OUTPUT != null" before generating a pdf

[yatzek]

Description

Check if "CHART_OUTPUT" command line parameter was provided before generating a pdf report.

This is to match the logic in the "customCommandLineValidation" method which checks if R is installed
when "CHART_OUTPUT != null"
This can be helpful when running on environments that do not have or cannot have R installed.
You should be able to use this tool on such environments provided you do not ask for a pdf report.

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Content

• Added or modified tests to cover changes and any new functionality
• Edited the README / documentation (if applicable)
• All tests passing on Travis

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[lbergelson]

@yatzek I fully support this plan. R is a constant irritation and we shouldn't require it when it's not absolutely necessary. I'm not sure this accomplishes the goal though. There are additional checks in some of those classes to make sure that the chart output file is writeable which will crash if you pass null through even with your change.

It's also a bit weird to have to pass null in an argument at all, maybe making CHART optional would make more sense? I guess people will forget it then though when they meant to include it.

[kachulis]

@yatzek will you be able to respond to @lbergelson's comments?

[droazen]

@yatzek It seems that there are a couple of outstanding issues that need to be resolved before we could accept this PR:

• CHART_OUTPUT should be made an optional argument
• There are a couple additional usages of CHART_OUTPUT not caught by this PR that need null checks in order for this to work
• There needs to be a test that demonstrates that the metrics collection can run successfully when CHART_OUTPUT is not specified.

Please let us know if you're able to make these changes