Remove is_null() calls and use require_once
Remove is_null() calls and use require_once

* Change is_null() in "=== null" to be consistant with "!== null" in
codebase (as a bonus, it's faster (no function call))
* Use require_once instead of require (also marginally faster)
* Remove parenthesis from require_once calls, as it's a language
construct
* Use null coalescing operator to improve readability


LitPHP/lit