basic/escape: add mode where empty arguments are still shown as ""

For variables, FOO= is OK. But when quoting positional arguments, we want to
use something with quotes ("", '', or even $'') for an empty string.

src/basic/escape.c

@@ -504,6 +504,9 @@ char* shell_maybe_quote(const char *s, ShellEscapeFlags flags) {
          * string. Note that we treat benign UTF-8 characters as needing
          * escaping too, but that should be OK. */
 
+        if (FLAGS_SET(flags, SHELL_ESCAPE_EMPTY) && isempty(s))
+                return strdup("\"\""); /* We don't use $'' here in the POSIX mode. "" is fine too. */
+
         for (p = s; *p; p++)
                 if (*p <= ' ' ||
                     *p >= 127 ||

src/basic/escape.h

@@ -38,6 +38,7 @@ typedef enum ShellEscapeFlags {
          * Tabs and newlines are escaped. */
 
         SHELL_ESCAPE_POSIX = 1 << 1, /* Use POSIX shell escape syntax (a string enclosed in $'') instead of plain quotes. */
+        SHELL_ESCAPE_EMPTY = 1 << 2, /* Format empty arguments as "". */
 } ShellEscapeFlags;
 
 char* cescape(const char *s);