Avoid preloading empty href attributes with Optimize critical images

[camilamadronero]

Describe the bug
The Optimize critical images feature adds a preload tag for empty href values.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://neo.law/ or any site with a markup such as <div style="background-image:url();"></div> as a markup for LCP element.
2. Run Optimize critical images on it.
3. See the resulting preload markup: <link rel="preload" data-rocket-preload as="image" href="" fetchpriority="high">

Expected behavior
Optimize critical images should avoid adding a preload tag in these conditions.

Screenshots
Original markup: https://jmp.sh/34iunv8H
Preload tag: https://jmp.sh/y6BkzWZq

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Ticket: https://secure.helpscout.net/conversation/2762969880/523284?viewId=273761
Slack thread: https://wp-media.slack.com/archives/C072P5EU5DF/p1731706632758679

Thanks!