sd-json: Introduce SD_JSON_FORMAT_EMPTY_OBJECT

This mirrors SD_JSON_FORMAT_EMPTY_ARRAY

We also removed the assertion that a non-null variant is passed into
sd_json_variant_format. It _is_ actually valid to pass a null variant
through to sd_json_variant_dump. If neither SD_JSON_FORMAT_EMPTY_* flag
is set, then sd_json_variant_format will simply return an empty string.

The removal of the assertion fixes a crash in 'varlinkctl --exec call'

(cherry picked from commit 7e70352bf10c2fad0099f69e0c451ff1ce8dc362)

src/libsystemd/sd-json/sd-json.c

@@ -1868,7 +1868,6 @@ _public_ int sd_json_variant_format(sd_json_variant *v, sd_json_format_flags_t f
         /* Returns the length of the generated string (without the terminating NUL),
          * or negative on error. */
 
-        assert_return(v, -EINVAL);
         assert_return(ret, -EINVAL);
 
         if (!sd_json_format_enabled(flags))
@@ -1893,6 +1892,8 @@ _public_ int sd_json_variant_dump(sd_json_variant *v, sd_json_format_flags_t fla
         if (!v) {
                 if (flags & SD_JSON_FORMAT_EMPTY_ARRAY)
                         v = JSON_VARIANT_MAGIC_EMPTY_ARRAY;
+                else if (flags & SD_JSON_FORMAT_EMPTY_OBJECT)
+                        v = JSON_VARIANT_MAGIC_EMPTY_OBJECT;
                 else
                         return 0;
         }

src/systemd/sd-json.h

@@ -144,6 +144,7 @@ __extension__ typedef enum _SD_ENUM_TYPE_S64(sd_json_format_flags_t) {
         SD_JSON_FORMAT_FLUSH            = 1 << 9,  /* call fflush() after dumping JSON */
         SD_JSON_FORMAT_EMPTY_ARRAY      = 1 << 10, /* output "[]" for empty input */
         SD_JSON_FORMAT_CENSOR_SENSITIVE = 1 << 11, /* replace all sensitive elements with the string "<sensitive data>" */
+        SD_JSON_FORMAT_EMPTY_OBJECT     = 1 << 12, /* output "{}" for empty input */
         _SD_ENUM_FORCE_S64(JSON_FORMAT_FLAGS)
 } sd_json_format_flags_t;