fix(usage): accept tenant/metric id "0" in collect()

empty() treats the string "0" as empty, so a tenant or metric literally
named "0" was wrongly rejected. Compare against '' instead.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: loks0n

src/Usage/Accumulator.php

@@ -53,10 +53,12 @@ public function __construct(Usage $usage)
      */
     public function collect(string $tenant, string $metric, int $value, string $type, array $tags = []): self
     {
-        if (empty($tenant)) {
+        // Compare against '' rather than empty(): the string "0" is a valid
+        // tenant/metric id but empty("0") is true in PHP.
+        if ($tenant === '') {
             throw new \InvalidArgumentException('Tenant cannot be empty');
         }
-        if (empty($metric)) {
+        if ($metric === '') {
             throw new \InvalidArgumentException('Metric name cannot be empty');
         }
         if ($value < 0) {

tests/Usage/AccumulatorTest.php

@@ -236,6 +236,17 @@ public function testEmptyTenantThrows(): void
         $this->accumulator->collect('', 'requests', 10, Usage::TYPE_EVENT);
     }
 
+    public function testTenantZeroIsAccepted(): void
+    {
+        // "0" is a valid tenant id even though empty("0") is true in PHP
+        $this->accumulator->collect('0', 'requests', 10, Usage::TYPE_EVENT);
+
+        $this->assertEquals(1, $this->accumulator->count());
+
+        $this->assertTrue($this->accumulator->flush());
+        $this->assertEquals('0', $this->adapter->batches[0]['metrics'][0]['tenant']);
+    }
+
     public function testEmptyMetricNameThrows(): void
     {
         $this->expectException(\InvalidArgumentException::class);