[chart.js] Add return type null to NestedTickOptions callback (#43143)

* [chart.js] Add return type null to NestedTickOptions callback

* [chart.js] Add undefined as callback return value

types/chart.js/chart.js-tests.ts

@@ -68,7 +68,21 @@ const chart: Chart = new Chart(ctx, {
             xAxes: [
                 {
                     ticks: {
-                        callback: Math.floor,
+                        callback: (value) => {
+                            if (value === 10) {
+                                return Math.floor(value);
+                            }
+
+                            if (value === 20) {
+                                return `${value}`;
+                            }
+
+                            if (value === 30) {
+                                return undefined;
+                            }
+
+                            return null;
+                        },
                         sampleSize: 10,
                     },
                     gridLines: {

types/chart.js/index.d.ts

@@ -562,7 +562,10 @@ declare namespace Chart {
         backdropPaddingX?: number;
         backdropPaddingY?: number;
         beginAtZero?: boolean;
-        callback?(value: any, index: any, values: any): string | number;
+        /**
+         * If the callback returns null or undefined the associated grid line will be hidden.
+         */
+        callback?(value: number | string, index: number, values: number[] | string[]): string | number | null | undefined;
         display?: boolean;
         fontColor?: ChartColor;
         fontFamily?: string;