chore(php): fix php 8.5 deprecations 33.x (#25714)

* chore(php): prevent PHP 8.5 null array key deprecation

* chore(php): fix chr() deprecation in CodedOutputStream.php

Author: bshaffer

php/src/Google/Protobuf/Internal/CodedOutputStream.php

@@ -98,7 +98,7 @@ public static function writeVarintToArray($value, &$buffer, $trim = false)
         }
 
         while (($low >= 0x80 || $low < 0) || $high != 0) {
-            $buffer[$current] = chr($low | 0x80);
+            $buffer[$current] = chr(($low | 0x80) & 0xFF);
             $value = ($value >> 7) & ~(0x7F << ((PHP_INT_SIZE << 3) - 7));
             $carry = ($high & 0x7F) << ((PHP_INT_SIZE << 3) - 7);
             $high = ($high >> 7) & ~(0x7F << ((PHP_INT_SIZE << 3) - 7));

php/src/Google/Protobuf/Internal/Descriptor.php

@@ -54,7 +54,7 @@ public function getFullName()
     public function addField($field)
     {
         $this->field[$field->getNumber()] = $field;
-        $this->json_to_field[$field->getJsonName()] = $field;
+        $this->json_to_field[$field->getJsonName() ?? ''] = $field;
         $this->name_to_field[$field->getName()] = $field;
         $this->index_to_field[] = $field;
     }

php/src/Google/Protobuf/Internal/DescriptorPool.php

@@ -75,8 +75,8 @@ public function addDescriptor($descriptor)
         $this->unique_descs[$descriptor->getFullName()] =
             $descriptor;
         $this->class_to_desc[$descriptor->getClass()] = $descriptor;
-        $this->class_to_desc[$descriptor->getLegacyClass()] = $descriptor;
-        $this->class_to_desc[$descriptor->getPreviouslyUnreservedClass()] = $descriptor;
+        $this->class_to_desc[$descriptor->getLegacyClass() ?? ''] = $descriptor;
+        $this->class_to_desc[$descriptor->getPreviouslyUnreservedClass() ?? ''] = $descriptor;
         foreach ($descriptor->getNestedType() as $nested_type) {
             $this->addDescriptor($nested_type);
         }
@@ -90,7 +90,7 @@ public function addEnumDescriptor($descriptor)
         $this->proto_to_class[$descriptor->getFullName()] =
             $descriptor->getClass();
         $this->class_to_enum_desc[$descriptor->getClass()] = $descriptor;
-        $this->class_to_enum_desc[$descriptor->getLegacyClass()] = $descriptor;
+        $this->class_to_enum_desc[$descriptor->getLegacyClass() ?? ''] = $descriptor;
     }
 
     public function getDescriptorByClassName($klass)