Adds check for empty thousands separator

[kjeldahl]

A check is added for empty thousands separator so it is possible to render numbers without a thousands separator
#914

[kjeldahl]

@nicolaskruchten I was unable to compile and test the patch due to some node setup issues.

[co-operate]

+1
Had the same issue, came to the same solution. In pivot.js this will result in:

if (thousandsSep) {
  while (rgx.test(x1)) {
    x1 = x1.replace(rgx, '$1' + thousandsSep + '$2');
  }
}