Tokenizers/Comment: bug fix - empty docblock

This commit fixes an edge case tokenizer bug, where a - completely empty, not even whitespace - _DocBlock_, would not be tokenized correctly.

Without this commit, the `/***/` code snippet was tokenized as:
```
 13 | L10 | C  1 | CC 0 | ( 0) | T_DOC_COMMENT_OPEN_TAG     | [  5]: /***/
 14 | L10 | C  6 | CC 0 | ( 0) | T_DOC_COMMENT_CLOSE_TAG    | [  0]:
```

With the fix applied, it will be tokenized as:
```
 13 | L10 | C  1 | CC 0 | ( 0) | T_DOC_COMMENT_OPEN_TAG     | [  3]: /**
 14 | L10 | C  4 | CC 0 | ( 0) | T_DOC_COMMENT_CLOSE_TAG    | [  2]: */
```

src/Tokenizers/Comment.php

@@ -41,9 +41,16 @@ public function tokenizeString($string, $eolChar, $stackPtr)
             extra star when they are used for function and class comments.
         */
 
-        $char    = ($numChars - strlen(ltrim($string, '/*')));
-        $openTag = substr($string, 0, $char);
-        $string  = ltrim($string, '/*');
+        $char      = ($numChars - strlen(ltrim($string, '/*')));
+        $lastChars = substr($string, -2);
+        if ($char === $numChars && $lastChars === '*/') {
+            // Edge case: docblock without whitespace or contents.
+            $openTag = substr($string, 0, -2);
+            $string  = $lastChars;
+        } else {
+            $openTag = substr($string, 0, $char);
+            $string  = ltrim($string, '/*');
+        }
 
         $tokens[$stackPtr] = [
             'content'      => $openTag,

tests/Core/Tokenizer/Comment/SingleLineDocBlockTest.inc

@@ -3,6 +3,9 @@
 /* testEmptyBlockCommentNoWhiteSpace */
 /**/
 
+/* testEmptyDocblockNoWhiteSpace */
+/***/
+
 /* testEmptyDocblockWithWhiteSpace */
 /** */
 

tests/Core/Tokenizer/Comment/SingleLineDocBlockTest.php

@@ -28,6 +28,11 @@ final class SingleLineDocBlockTest extends CommentTestCase
     public static function dataDocblockOpenerCloser()
     {
         return [
+            'Single line docblock: empty, no whitespace'  => [
+                'marker'       => '/* testEmptyDocblockNoWhiteSpace */',
+                'closerOffset' => 1,
+                'expectedTags' => [],
+            ],
             'Single line docblock: only whitespace'       => [
                 'marker'       => '/* testEmptyDocblockWithWhiteSpace */',
                 'closerOffset' => 2,
@@ -79,12 +84,31 @@ public function testEmptyBlockCommentNoWhiteSpace()
 
 
     /**
-     * Verify tokenization of an empty, single line DocBlock.
+     * Verify tokenization of an empty, single line DocBlock without whitespace between the opener and closer.
      *
      * @phpcs:disable Squiz.Arrays.ArrayDeclaration.SpaceBeforeDoubleArrow -- Readability is better with alignment.
      *
      * @return void
      */
+    public function testEmptyDocblockNoWhiteSpace()
+    {
+        $expectedSequence = [
+            [T_DOC_COMMENT_OPEN_TAG   => '/**'],
+            [T_DOC_COMMENT_CLOSE_TAG  => '*/'],
+        ];
+
+        $target = $this->getTargetToken('/* '.__FUNCTION__.' */', T_DOC_COMMENT_OPEN_TAG);
+
+        $this->checkTokenSequence($target, $expectedSequence);
+
+    }//end testEmptyDocblockNoWhiteSpace()
+
+
+    /**
+     * Verify tokenization of an empty, single line DocBlock.
+     *
+     * @return void
+     */
     public function testEmptyDocblockWithWhiteSpace()
     {
         $expectedSequence = [