Bug 1853920 - [2/2] quote empty strings in Windows command lines r=win-reviewers,gstoll

Quote empty arguments in command lines. Add appropriate tests.

Differential Revision: https://phabricator.services.mozilla.com/D191128

toolkit/xre/CmdLineAndEnvUtils.h

@@ -362,8 +362,12 @@ inline size_t CopyArgImpl_(wchar_t* d, const wchar_t* s) {
   };
 
   bool hasDoubleQuote = wcschr(s, L'"') != nullptr;
-  // Only add doublequotes if the string contains a space or a tab
-  bool addDoubleQuotes = wcspbrk(s, kCommandLineDelimiter) != nullptr;
+  // Only add doublequotes if...
+  bool addDoubleQuotes =
+      // ... the string is empty, or...
+      *s == '\0' ||
+      // ... the string contains a space or a tab.
+      wcspbrk(s, kCommandLineDelimiter) != nullptr;
 
   if (addDoubleQuotes) {
     appendChar('"');

toolkit/xre/test/win/TestXREMakeCommandLineWin.ini

@@ -92,3 +92,12 @@ output_24=a:\b "c\アルファ オメガ\d"
 
 input_25=アルファ オメガ
 output_25=アルファ オメガ
+
+input_26=arg1 "" arg3
+output_26=arg1 "" arg3
+
+input_27=""""
+output_27=""""
+
+input_28=""" a """
+output_28=""" a """