PHPStan Level 1

[lloc]

Setting PHPStan to level 1 revealed three minor issues, which have been addressed in this PR:

• Ignored the warning: "Constant WP_CONTENT_URL not found."
• Fixed: "Variable $slug in empty() always exists and is not falsy." ( array_filter has been applied before.)
• Fixed: "Variable $rows might not be defined."

[lloc]

@JJJ @spacedmonkey Do you think we can get those changes in? I would like to go ahead and raise the level. ;-)