Fixes goBack() behavior (#2996)

Task/Issue URL: https://app.asana.com/0/414709148257752/1207663375808651/f

Description:

Fixes an issue caused by incomplete logic for the goBack() behavior when implementing DuckPlayer
Cleans up the goBack() method for clarity and early returns (remove if/else)

DuckDuckGo/TabViewController.swift

@@ -724,23 +724,30 @@ class TabViewController: UIViewController {
     func goBack() {
         dismissJSAlertIfNeeded()
 
-        if let handler = youtubeNavigationHandler {
+        if let url = url, url.isDuckPlayer, let handler = youtubeNavigationHandler {
             handler.goBack(webView: webView)
             chromeDelegate?.omniBar.resignFirstResponder()
-        } else {
-            if isError {
-                hideErrorMessage()
-                url = webView.url
-                onWebpageDidStartLoading(httpsForced: false)
-                onWebpageDidFinishLoading()
-            } else if webView.canGoBack {
-                webView.goBack()
-                chromeDelegate?.omniBar.resignFirstResponder()
-            } else if openingTab != nil {
-                delegate?.tabDidRequestClose(self)
-            }
+            return
+        }
+
+        if isError {
+            hideErrorMessage()
+            url = webView.url
+            onWebpageDidStartLoading(httpsForced: false)
+            onWebpageDidFinishLoading()
+            return
+        }
+
+        if webView.canGoBack {
+            webView.goBack()
+            chromeDelegate?.omniBar.resignFirstResponder()
+            return
         }
 
+        if openingTab != nil {
+            delegate?.tabDidRequestClose(self)
+        }
+
     }
 
     func goForward() {