Fixed passing null to strpos/strrpos/substr etc

Summary:
in PHP 8.1+, passing null to strpos/strrpos/substr etc. emits deprecation warnings, and PHP 9 will make this a fatal error. If this SDK ever targets PHP 9+, you'd want a guard

Per PHP version:

PHP 8.0: silent coercion (null → ""), returns false. No warning.
PHP 8.1 – 8.x: emits an E_DEPRECATED warning, still returns false. Just a log entry, execution continues.
PHP 9.0 (planned): throws an uncaught TypeError. Execution halts at the call site and propagates up the stack — same observable behavior as the JS TypeError we just fixed.
So the fix calculus changes once PHP 9 is in play:

Today (PHP 8.x at Meta): no crash, no fix needed
PHP 9: same crash class as the Node.js bug — would need a similar early-return guard in computeETLDPlus1ForHost
If the team plans to upgrade, applying the analogous PHP guard now would future-proof.

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overriding_review_checks_triggers_an_audit_and_retroactive_review
Oncall Short Name: advertiser_experience_foundations

Differential Revision: D104760726

fbshipit-source-id: 8665505c6b91a81f440ade4a90e58779a65a169f

Author: Jiaming You

php/capi-param-builder/src/ParamBuilder.php

@@ -345,6 +345,11 @@ private function computeETLDPlus1ForHost($host)
         if ($this->etld_plus_1 === null || $this->host !== $host) {
             // in case a new host is passed in for the same request
             $this->host = $host;
+            if ($host === null || $host === '') {
+                $this->etld_plus_1 = null;
+                $this->sub_domain_index = 0;
+                return;
+            }
             $host = ParamBuilder::extractHostFromHttpHost($host);
 
             if (