bacdcode.c : Fix ability to write empty string (#871)

src/bacnet/bacdcode.c

@@ -2487,6 +2487,8 @@ int bacnet_character_string_decode(
                 string_value = (const char *)&apdu[1];
                 (void)characterstring_init(
                     char_string, encoding, string_value, len_value - 1);
+            } else {
+                (void)characterstring_init(char_string, encoding, NULL, 0);
             }
             len = (int)len_value;
         }