fix handling of empty sequence items

When a line contains only a dash it cannot safely be assumed that
it contains a nested list or an embedded mapping. If the next line
starts with a dash at the same indentation, the current line's item
is to be treated as `null`.

src/Symfony/Component/Yaml/Parser.php

@@ -93,7 +93,7 @@ class Parser
                     $c = $this->getRealCurrentLineNb() + 1;
                     $parser = new Parser($c);
                     $parser->refs =& $this->refs;
-                    $data[] = $parser->parse($this->getNextEmbedBlock(), $exceptionOnInvalidType, $objectSupport);
+                    $data[] = $parser->parse($this->getNextEmbedBlock(null, true), $exceptionOnInvalidType, $objectSupport);
                 } else {
                     if (isset($values['leadspaces'])
                         && ' ' == $values['leadspaces']
@@ -281,15 +281,20 @@ class Parser
     /**
      * Returns the next embed block of YAML.
      *
-     * @param int     $indentation The indent level at which the block is to be read, or null for default
+     * @param int  $indentation The indent level at which the block is to be read, or null for default
+     * @param bool $inSequence  True if the enclosing data structure is a sequence
      *
      * @return string A YAML string
      *
      * @throws ParseException When indentation problem are detected
      */
-    private function getNextEmbedBlock($indentation = null)
+    private function getNextEmbedBlock($indentation = null, $inSequence = false)
     {
-        $this->moveToNextLine();
+        $oldLineIndentation = $this->getCurrentLineIndentation();
+
+        if (!$this->moveToNextLine()) {
+            return;
+        }
 
         if (null === $indentation) {
             $newIndent = $this->getCurrentLineIndentation();
@@ -305,6 +310,14 @@ class Parser
 
         $data = array(substr($this->currentLine, $newIndent));
 
+        if ($inSequence && $oldLineIndentation === $newIndent && '-' === $data[0][0]) {
+            // the previous line contained a dash but no item content, this line is a sequence item with the same indentation
+            // and therefore no nested list or mapping
+            $this->moveToPreviousLine();
+
+            return;
+        }
+
         $isItUnindentedCollection = $this->isStringUnIndentedCollectionItem($this->currentLine);
 
         // Comments must not be removed inside a string block (ie. after a line ending with "|")

src/Symfony/Component/Yaml/Tests/Fixtures/YtsBasicTests.yml

@@ -11,6 +11,30 @@ yaml: |
 php: |
     array('apple', 'banana', 'carrot')
 ---
+test: Sequence With Item Being Null In The Middle
+brief: |
+    You can specify a list in YAML by placing each
+    member of the list on a new line with an opening
+    dash. These lists are called sequences.
+yaml: |
+    - apple
+    -
+    - carrot
+php: |
+    array('apple', null, 'carrot')
+---
+test: Sequence With Last Item Being Null
+brief: |
+    You can specify a list in YAML by placing each
+    member of the list on a new line with an opening
+    dash. These lists are called sequences.
+yaml: |
+    - apple
+    - banana
+    -
+php: |
+    array('apple', 'banana', null)
+---
 test: Nested Sequences
 brief: |
     You can include a sequence within another