LineChart error on empty data

[MicahKimel]

I am dynamically setting the data labels, datasets, and legend, but was experiencing some errors when the data was empty such as the LinearGradient in AbstractChart.js line 192 can't read property length of undefined. I was able to fix this by writing some inline if statements to check for nulls in the AbstractChart.js and the LineChart.js of my dist. If this is something that would be good to merge into the project let me know.

[MicahKimel]

Here are the conditional statements I wrote to fix this issue

#667