Avoid warning when writing an empty string with eio

No warning emitted from uv, but there's also no reason to call uv_fs_write().

lib/EioHandle.php

@@ -132,6 +132,10 @@ class EioHandle implements Handle
     {
         $length = \strlen($data);
 
+        if ($length === 0) {
+            return new Success(0);
+        }
+
         $deferred = new Deferred;
         $this->poll->listen($deferred->promise());
 

lib/UvHandle.php

@@ -150,6 +150,10 @@ class UvHandle implements Handle
     {
         $length = \strlen($data);
 
+        if ($length === 0) {
+            return new Success(0);
+        }
+
         $deferred = new Deferred;
         $this->poll->listen($deferred->promise());
 

test/HandleTest.php

@@ -41,6 +41,21 @@ abstract class HandleTest extends TestCase
         });
     }
 
+    public function testEmptyWrite()
+    {
+        $this->execute(function () {
+            $path = Fixture::path() . "/write";
+
+            $handle = yield File\open($path, "c+");
+            $this->assertSame(0, $handle->tell());
+
+            yield $handle->write("");
+            $this->assertSame(0, $handle->tell());
+
+            yield $handle->close();
+        });
+    }
+
     public function testWriteAfterClose()
     {
         $this->execute(function () {