process: adjust quote_cmd_arg for empty arguments

adjust quote_cmd_arg so that empty command line
arguments are double quoted instead of skipped

src/win/process.c

@@ -430,11 +430,10 @@ WCHAR* quote_cmd_arg(const WCHAR *source, WCHAR *target) {
   int quote_hit;
   WCHAR* start;
 
-  /*
-   * Check if the string must be quoted;
-   * if unnecessary, don't do it, it may only confuse older programs.
-   */
   if (len == 0) {
+    /* Need double quotation for empty argument */
+    *(target++) = L'"';
+    *(target++) = L'"';
     return target;
   }
 

test/test-spawn.c

@@ -808,6 +808,7 @@ WCHAR* quote_cmd_arg(const WCHAR *source, WCHAR *target);
 
 TEST_IMPL(argument_escaping) {
   const WCHAR* test_str[] = {
+    L"",
     L"HelloWorld",
     L"Hello World",
     L"Hello\"World",