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);

src/test/test-escape.c

@@ -140,7 +140,9 @@ static void test_shell_maybe_quote_one(const char *s, ShellEscapeFlags flags, co
 static void test_shell_maybe_quote(void) {
 
         test_shell_maybe_quote_one("", 0, "");
+        test_shell_maybe_quote_one("", SHELL_ESCAPE_EMPTY, "\"\"");
         test_shell_maybe_quote_one("", SHELL_ESCAPE_POSIX, "");
+        test_shell_maybe_quote_one("", SHELL_ESCAPE_POSIX | SHELL_ESCAPE_EMPTY, "\"\"");
         test_shell_maybe_quote_one("\\", 0, "\"\\\\\"");
         test_shell_maybe_quote_one("\\", SHELL_ESCAPE_POSIX, "$'\\\\'");
         test_shell_maybe_quote_one("\"", 0, "\"\\\"\"");
@@ -156,7 +158,9 @@ static void test_shell_maybe_quote(void) {
         test_shell_maybe_quote_one("foo \"bar\" waldo", 0, "\"foo \\\"bar\\\" waldo\"");
         test_shell_maybe_quote_one("foo \"bar\" waldo", SHELL_ESCAPE_POSIX, "$'foo \"bar\" waldo'");
         test_shell_maybe_quote_one("foo$bar", 0, "\"foo\\$bar\"");
+        test_shell_maybe_quote_one("foo$bar", SHELL_ESCAPE_EMPTY, "\"foo\\$bar\"");
         test_shell_maybe_quote_one("foo$bar", SHELL_ESCAPE_POSIX, "$'foo$bar'");
+        test_shell_maybe_quote_one("foo$bar", SHELL_ESCAPE_POSIX | SHELL_ESCAPE_EMPTY, "$'foo$bar'");
 
         /* Note that current users disallow control characters, so this "test"
          * is here merely to establish current behaviour. If control characters