Adds a null check

[arunshenoy99]

Proposed changes

Type of Change

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

• [ ] I have read the CONTRIBUTING doc
• [ ] Linting and tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)

Further comments

Deprecated: md5): Passing null to parameter #1 ($string) of type string is deprecated in /app/wordpress/content/mu- plugins/endurance-page-cache.php on line 763

[arunshenoy99]

@wpscholar please check if this makes sense. Wasn't able to replicate why $uri would be null.