Fix Windows test regression from commit 149250a

Commit: 149250ab92feadd9fe045b54d6e42f33d185a595

ProcessExecutor::escape handled a false value inconsistently across
platforms, returning an emtpy string on Windows, otherwise `''`. This
is fixed to return `""` on Windows.

The GitDownloaderTest code has been appropriately updated.

src/Composer/Util/ProcessExecutor.php

@@ -155,7 +155,7 @@ class ProcessExecutor
         //@see https://bugs.php.net/bug.php?id=43784
         //@see https://bugs.php.net/bug.php?id=49446
         if ('\\' === DIRECTORY_SEPARATOR) {
-            if ('' === $argument) {
+            if ((string) $argument === '') {
                 return escapeshellarg($argument);
             }
 

tests/Composer/Test/Downloader/GitDownloaderTest.php

@@ -734,7 +734,7 @@ composer https://github.com/old/url (push)
             $cmd = str_replace('cd ', 'cd /D ', $cmd);
             $cmd = str_replace('composerPath', getcwd().'/composerPath', $cmd);
 
-            return str_replace('""', '', strtr($cmd, "'", '"'));
+            return strtr($cmd, "'", '"');
         }
 
         return $cmd;