Fix NULL values handling in nyc data

[szymanskir]

This is a patch regarding #96 . I looked around the code and noticed the part where \N (the previous way how missing values where marked in the nyc data) characters are removed. I added lines removing "NULL" and this seems to be fixing the issue :D

[mpadge]

Thanks @szymanskir - that's a good catch and looks like a great fix. I'll check it out early next week, and expect to merge then. Thank you for using the package, and for your input here

[mpadge]

@szymanskir all looks good, but could you please move the new line to a single instance outside the if clause, and so have just a single line to replace all "NULL" with empty strings? Could you maybe also add a comment referencing issue#96, and explaining that these "NULL" values arise only in NYC data from Aug 2018 onwards? Happy to merge once that has been done.

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Note also that the travis fail is not related to your PR, so you may ignore that (it's related to #95)

[mpadge]

One more thought: The boost::replace_all call comes with a time cost, so it'd probably also be better to only run it where needed by first having an extra line of if (utils::strfound (city, "ny")) - this is equivalent to if (city == "ny"), but string equality in C++ doesn't work like that, so these comparisons use a safer, locally-defined strfound function.

[szymanskir]

@mpadge thank you for the review. I have modified the code according to your remarks.

[mpadge]

One more thing @szymanskir: We aim to acknowledge all contributions and all contributors here, so would you please be so kind as to add your name to the DESCRIPTION file with role = "ctb" (after Tom Buckley)? If possible, could you also run codemetar::write_codemeta() to update the metadata file? Thanks!