Generic/ScopeIndent: bug fix for debug mode

Prevent an "Undefined array index" PHP notice when running in debug mode when there are no scopes left.

Issue can be reproduced by running:
```bash
phpcs ./src/Runner.php --runtime-set scope_indent_debug 1
```

Author: jrfnl

src/Standards/Generic/Sniffs/WhiteSpace/ScopeIndentSniff.php

@@ -644,7 +644,7 @@ public function process(File $phpcsFile, $stackPtr)
                 }
 
                 $conditionToken = array_pop($openScopes);
-                if ($this->debug === true) {
+                if ($this->debug === true && $conditionToken !== null) {
                     $line = $tokens[$conditionToken]['line'];
                     $type = $tokens[$conditionToken]['type'];
                     echo "\t=> removed open scope $conditionToken ($type) on line $line".PHP_EOL;