Empty string as X value causes "Cannot read property `toString` of undefined

[DonaldAlan]

I'm submitting a ... (check one with "x")

• [x] bug report - search github for a similar issue or PR before submitting
• [ ] feature request
• [ ] support request - use StackOverflow (add the ngx-charts tag) or the gitter chat for support questions

Current behavior

If I use the empty string (''), which has truthiness of false, as the X value in a chart (e.g., a Tree Map), ColorHelper.push generates a 'ERROR TypeError: Cannot read property 'toString' of undefined."

Expected behavior

Empty strings should be allowed as X. Reproduction of the problem

What is the motivation / use case for changing the behavior?

Please tell us about your environment:

• ngx-charts version: x.x.x

  10.0.0

• Angular version: x.x.x

  7.1.4

• Browser: [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]

• Language: [all | TypeScript X.X | ES6/7 | ES5]

[chriszrc]

total bug for sure, I use " " (note the space) in various places in ngx-charts to circumvent these issues (which should get addressed)