Ensure XMLSerializer can deal with empty token collections

src/XMLSerializer.php

@@ -57,12 +57,15 @@ class XMLSerializer {
         $this->writer->startDocument();
         $this->writer->startElement('source');
         $this->writer->writeAttribute('xmlns', $this->xmlns->asString());
-        $this->writer->startElement('line');
-        $this->writer->writeAttribute('no', '1');
 
-        $this->previousToken = $tokens[0];
-        foreach ($tokens as $token) {
-            $this->addToken($token);
+        if (count($tokens) > 0) {
+            $this->writer->startElement('line');
+            $this->writer->writeAttribute('no', '1');
+
+            $this->previousToken = $tokens[0];
+            foreach ($tokens as $token) {
+                $this->addToken($token);
+            }
         }
 
         $this->writer->endElement();

tests/XMLSerializerTest.php

@@ -40,4 +40,11 @@ class XMLSerializerTest extends TestCase {
         $this->assertEquals($expected, $serializer->toXML($this->tokens));
     }
 
+    public function testEmptyCollectionCreatesEmptyDocument() {
+        $expected = file_get_contents(__DIR__ . '/_files/empty.xml');
+
+        $serializer = new XMLSerializer();
+        $this->assertEquals($expected, $serializer->toXML((new TokenCollection())));
+    }
+
 }

tests/_files/empty.xml

+<?xml version="1.0"?>
+<source xmlns="https://github.com/theseer/tokenizer"/>