fix(material/slider): make value non-nullable (#22912)

Makes the value of the slider a `number`, rather than `number | null` in order to make it easier to use.

Fixes #22444.

(cherry picked from commit 71afc46)

Author: crisbeto

src/material/slider/slider.ts

@@ -190,11 +190,6 @@ export class MatSlider extends _MatSliderMixinBase
   get min(): number { return this._min; }
   set min(v: number) {
     this._min = coerceNumberProperty(v, this._min);
-
-    // If the value wasn't explicitly set by the user, set it to the min.
-    if (this._value === null) {
-      this.value = this._min;
-    }
     this._percent = this._calculatePercentage(this._value);
 
     // Since this also modifies the percentage, we need to let the change detection know.
@@ -242,16 +237,16 @@ export class MatSlider extends _MatSliderMixinBase
 
   /** Value of the slider. */
   @Input()
-  get value(): number | null {
+  get value(): number {
     // If the value needs to be read and it is still uninitialized, initialize it to the min.
     if (this._value === null) {
       this.value = this._min;
     }
-    return this._value;
+    return this._value as number;
   }
-  set value(v: number | null) {
+  set value(v: number) {
     if (v !== this._value) {
-      let value = coerceNumberProperty(v);
+      let value = coerceNumberProperty(v, 0);
 
       // While incrementing by a decimal we can end up with values like 33.300000000000004.
       // Truncate it to ensure that it matches the label and to make it easier to work with.

tools/public_api_guard/material/slider.d.ts

@@ -23,8 +23,8 @@ export declare class MatSlider extends _MatSliderMixinBase implements ControlVal
     set thumbLabel(value: boolean);
     get tickInterval(): 'auto' | number;
     set tickInterval(value: 'auto' | number);
-    get value(): number | null;
-    set value(v: number | null);
+    get value(): number;
+    set value(v: number);
     readonly valueChange: EventEmitter<number | null>;
     valueText: string;
     get vertical(): boolean;