[BUGFIX] Avoid unitialized variable in VersionToolbarItem (#1167)

Third argument $returnValue of CommandUtility::exec() has been
type hinted to int in [1]. bootstrap_package fails now since
it uses a not initialized variable. Init properly.

[1] https://forge.typo3.org/issues/97206

Classes/Backend/ToolbarItem/VersionToolbarItem.php

@@ -36,6 +36,7 @@ public function addVersionInformation(SystemInformationToolbarItem $systemInform
 
         // Try to get current version from git
         if (file_exists($extensionDirectory . '.git')) {
+            $returnCode = 0;
             CommandUtility::exec('git --version', $_, $returnCode);
             if ((int)$returnCode === 0) {
                 $currentDir = (string) getcwd();