fix(virtual-scroll): set this.approxItemHeight before update(true) is called first (#8350)

Author: ckaeslin

src/components/virtual-scroll/virtual-scroll.ts

@@ -343,17 +343,18 @@ export class VirtualScroll implements DoCheck, AfterContentInit, OnDestroy {
 
       this._init = true;
 
+      if (!this.approxItemHeight) {
+        this.approxItemHeight = '40px';
+        console.warn('Virtual Scroll: Please provide an "approxItemHeight" input to ensure proper virtual scroll rendering');
+      }
+
       this.update(true);
 
       this._platform.onResize(() => {
         console.debug('VirtualScroll, onResize');
         this.update(false);
       });
 
-      if (!this.approxItemHeight) {
-        this.approxItemHeight = '40px';
-        console.warn('Virtual Scroll: Please provide an "approxItemHeight" input to ensure proper virtual scroll rendering');
-      }
     }
   }