Checking if there is content in an iframe after pressing the back button in browser and going forward

I have a site that plays YouTube videos through an iframe. The site checks if the iframe has a source before allowing some actions. In the example below, the action is opening the iframe in full screen. If a user presses the back button in their browser with a source in the iframe and presses the forward button, the iframe contents load again but the iframe source is empty. How would I be able to check if there is content in the iframe? Note that for simplicity the code provided is shortened. Here is the original source.

document.getElementById("player").src = "";

function fullscreen() {
  var player = document.getElementById("player");
  if (player.src !== null) {
    if (player.requestFullscreen) {
    } else if (player.webkitRequestFullscreen) {
    } else if (player.msRequestFullscreen) {
    } else {
      alert("Unable to open video in full screen");
  } else {
    console.log("Error: unable to toggle full screen\nReason: no URL found");
<iframe id="player"></iframe>
<button id="full-screen">Open video in full screen</button>

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