Wait for placeholders to be present

Related issue:
- https://www.reddit.com/r/uBlockOrigin/comments/rgxlda/adblock_detected/

Related discussion:
- uBlockOrigin/uAssets@137070f#commitcomment-61964299

src/web_accessible_resources/googlesyndication_adsbygoogle.js

@@ -21,22 +21,32 @@
 
 (function() {
     'use strict';
-    window.adsbygoogle = {
-        loaded: true,
-        push: function() {
+    const init = ( ) => {
+        window.adsbygoogle = {
+            loaded: true,
+            push: function() {
+            }
+        };
+        const phs = document.querySelectorAll('.adsbygoogle');
+        const css = 'height:1px!important;max-height:1px!important;max-width:1px!important;width:1px!important;';
+        for ( let i = 0; i < phs.length; i++ ) {
+            const id = `aswift_${i}`;
+            if ( document.querySelector(`iframe#${id}`) !== null ) { continue; }
+            const fr = document.createElement('iframe');
+            fr.id = id;
+            fr.style = css;
+            const cfr = document.createElement('iframe');
+            cfr.id = `google_ads_frame${i}`;
+            fr.appendChild(cfr);
+            phs[i].appendChild(fr);
         }
     };
-    const phs = document.querySelectorAll('.adsbygoogle');
-    const css = 'height:1px!important;max-height:1px!important;max-width:1px!important;width:1px!important;';
-    for ( let i = 0; i < phs.length; i++ ) {
-        const id = `aswift_${i}`;
-        if ( document.querySelector(`iframe#${id}`) !== null ) { continue; }
-        const fr = document.createElement('iframe');
-        fr.id = id;
-        fr.style = css;
-        const cfr = document.createElement('iframe');
-        cfr.id = `google_ads_frame${i}`;
-        fr.appendChild(cfr);
-        phs[i].appendChild(fr);
+    if (
+        document.querySelectorAll('.adsbygoogle').length === 0 &&
+        document.readyState === 'loading'
+    ) {
+        window.addEventListener('DOMContentLoaded', init, { once: true });
+    } else {
+        init();
     }
 })();