Respect AspectRatio by making chart-dimensions nullable (#88)

Changes the width/height properties to be nullable. This is required for AspectRatio to work since it's ignored when height is set (and it's always set if it's not nullable). The defaults of 400/400 are also removed.

- As stated in the [responsive section](https://www.chartjs.org/docs/latest/general/responsive.html#configuration-options).

> Note that this option (AspectRatio) is ignored if the height is explicitly defined either as attribute or via the style.

src/ChartJs.Blazor/Charts/ChartBase.cs

@@ -25,13 +25,13 @@ public abstract class ChartBase<TConfig> : ComponentBase where TConfig : ConfigB
         /// The width of the canvas HTML element used to draw the chart.
         /// </summary>
         [Parameter]
-        public int Width { get; set; } = 400;
+        public int? Width { get; set; }
 
         /// <summary>
-        /// The height of the canvas HTML element used to draw the chart.
+        /// The height of the canvas HTML element used to draw the chart. Use null value when using AspectRatio.
         /// </summary>
         [Parameter]
-        public int Height { get; set; } = 400;
+        public int? Height { get; set; }
 
         /// <inheritdoc />
         protected override Task OnAfterRenderAsync(bool firstRender)