Nullable properties

[illided]

Allow list properties of function info be nullable implementing following behavior, which can help astminer run smoothly and be more predictable.

Condition	Consequence
Feature is not implemented	Exception thrown
Feature parsed badly	Null return (instead of immediate exception)
Feature parsed normally (without any exceptions)	Feature return

[SpirinEgor]

LGTM!
But could you also create documentation for FunctionInfo? Simple markdown file in docs/function_info.md where you can describe this behavior and in general how to work with it.

[illided]

LGTM!
But could you also create documentation for FunctionInfo? Simple markdown file in docs/function_info.md where you can describe this behavior and in general how to work with it.

I don't think that we need a whole markdown file just for this... Won't javadoc be enough?
I think it would be better to add some guide about how to add new language / parser in docs that could point in proper directions.

[SpirinEgor]

Fare enough. Let's create a doc file for the entire process of adding and supporting parsers/languages.