[6.x] Fix TypeError when asset last_modified meta is null

[v-Woody]

Summary

• Fixes a TypeError in Asset::lastModified() when the last_modified meta value is null
• Falls back to epoch 0 instead of passing null to Carbon::createFromTimestamp()
• Adds a test covering the null meta scenario

Fixes #14529

[jasonvarga]

Thank you.
I tweaked the test - it would've passed even without your fix.

And regarding your fix - it's a band-aid, but I'm fine with that for now.
e.g. It'll say last modified 55 years ago. At least it doesn't crash.