[Enhancement] Simplify chart label logic

[MadsBuchmann]

In the current version of Kirby, if a chart is created where an empty array is passed to the labels input property, we will auto generate blank labels to apply to the x-axis. Otherwise the chart will not render:

<kirby-chart [data]="[1,2,3]" [labels]="[]"></kirby-chart>

In #2082 it was however discovered that there exist cases where we want to be able to pass an empty array of labels to the chart. For example when utilizingtimeScale via customOptions. Here we want the default behaviour of chart.js - generating blank labels will result in them being appended to the x-axis, causing the chart to render incorrectly.

Logic was introduced in #2082 to make it possible to use a timeScale with the stock chart type. But to simplify this logic, i think we should simply always pass labels directly to chart.js if anything is given to the labels input property.

Only if nothing is passed in here should we have some default behaviour.

Pseudo-code:

if labels are given: return labels 
else if chart type is stock: return defaultStockChartLabels
else: return blank labels

This would be a breaking change as a chart now works with an empty array; it wouldn't after this change.

[MadsBuchmann]

Should be solved as part of #2108