fix: Failed to construct URL: Invalid URL at new URL (#31502)

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## **Description**
When go to certain websites the background console show error:
"TypeError: Failed to construct 'URL': Invalid URL at new URL."

Root Cause:
Sometimes the website's initiator is "null", not null, so we will create
URL object with Illegal string("null"):
![屏幕截图 2025-04-01
230234](https://github.com/user-attachments/assets/ce7ff859-19cb-471b-9804-13221b38e1df)


Solution:
Filter out "null" strings

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## **Related issues**

Fixes: #31465

## **Manual testing steps**

1. Open the browser
2. Open the background console
3. Go to https://soundcloud.com/
4. Check if the error "TypeError: Failed to construct 'URL': Invalid URL
at new URL" shown.

## **Screenshots/Recordings**

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### **Before**

![before](https://github.com/user-attachments/assets/fd681a83-e9c2-4bb2-b139-d8742f09d340)

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### **After**

![after](https://github.com/user-attachments/assets/ba4462c7-6cea-4f67-b4be-179f06c03231)
<!-- [screenshots/recordings] -->

## **Pre-merge author checklist**

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Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
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Co-authored-by: seaona <[email protected]>

Author: Teng-Web3

app/scripts/background.js

@@ -283,6 +283,7 @@ function maybeDetectPhishing(theController) {
       // is shipped.
       if (
         details.initiator &&
+        details.initiator !== 'null' &&
         // compare normalized URLs
         new URL(details.initiator).host === phishingPageUrl.host
       ) {