fix(material/schematics): support stricter TypeScript compiler flags

Fixes #17135

Author: Splaktar

src/material/schematics/ng-generate/table/files/__path__/__name@dasherize@if-flat__/__name@dasherize__-datasource.ts.template

@@ -103,6 +103,6 @@ export class <%= classify(name) %>DataSource extends DataSource<<%= classify(nam
 }
 
 /** Simple sort comparator for example ID/Name columns (for client-side sorting). */
-function compare(a, b, isAsc) {
+function compare(a: string | number, b: string | number, isAsc: boolean) {
   return (a < b ? -1 : 1) * (isAsc ? 1 : -1);
 }

src/material/schematics/ng-generate/tree/files/__path__/__name@dasherize@if-flat__/__name@dasherize__.component.ts.template

@@ -1,6 +1,5 @@
 import { Component<% if (!!viewEncapsulation) { %>, ViewEncapsulation<% }%><% if (changeDetection !== 'Default') { %>, ChangeDetectionStrategy<% }%> } from '@angular/core';
 import { MatTreeFlatDataSource, MatTreeFlattener } from '@angular/material/tree';
-import { of as observableOf } from 'rxjs';
 import { FlatTreeControl } from '@angular/cdk/tree';
 import { files } from './example-data';
 
@@ -84,7 +83,10 @@ export class <%= classify(name) %>Component {
   }
 
   /** Get the children for the node. */
-  getChildren(node: FileNode) {
-    return observableOf(node.children);
+  getChildren(node: FileNode): FileNode[] | null | undefined {
+    if (node.children === null || node.children === undefined) {
+      return node.children;
+    }
+    return node.children;
   }
 }