Fix parent id check, sampling cannot be tested here

Author: MilanGarnier

tests/ext/distributed_tracing/propagation_behavior_extract_ignore.phpt

@@ -19,7 +19,7 @@ $root = DDTrace\root_span();
 
 // fresh trace: not from upstream
 echo "same_as_upstream: " . ($root->traceId === "0000000000000000000000000000002a" ? "yes" : "no") . "\n";
-echo "parent_id: " . $root->parentId . "\n";
+echo "parent_id: " . ($root->parentId ?? 0) . "\n";
 
 // no span link (context discarded entirely)
 echo "links_count: " . count($root->links) . "\n";
@@ -28,15 +28,10 @@ echo "links_count: " . count($root->links) . "\n";
 $headers = DDTrace\generate_distributed_tracing_headers(['baggage']);
 echo "baggage: " . ($headers['baggage'] ?? 'none') . "\n";
 
-// upstream sampling priority not carried over
-$dd_headers = DDTrace\generate_distributed_tracing_headers(['datadog']);
-echo "sampling_priority: " . ($dd_headers['x-datadog-sampling-priority'] ?? 'none') . "\n";
-
 DDTrace\close_span();
 ?>
 --EXPECT--
 same_as_upstream: no
 parent_id: 0
 links_count: 0
 baggage: none
-sampling_priority: none