disallow empty string for baggage name

spec 1.36.0 clarifies that empty string is not a valid baggage name

Author: brettmc

src/API/Baggage/BaggageBuilder.php

@@ -22,6 +22,9 @@ public function remove(string $key): BaggageBuilderInterface
     /** @inheritDoc */
     public function set(string $key, $value, ?MetadataInterface $metadata = null): BaggageBuilderInterface
     {
+        if ($key === '') {
+            return $this;
+        }
         $metadata ??= Metadata::getEmpty();
 
         $this->entries[$key] = new Entry($value, $metadata);

tests/Unit/API/Baggage/BaggageTest.php

@@ -125,5 +125,11 @@ public function test_get_all(): void
         );
     }
 
+    public function test_empty_name_disallowed(): void
+    {
+        $baggage = Baggage::getBuilder()->set('', 'bar')->build();
+        $this->assertTrue($baggage->isEmpty());
+    }
+
     // endregion
 }