Don't attempt to escape null characters

src/api/variables.c

@@ -175,7 +175,7 @@ int escape_json_string(char *dest, int dl, char *src, int sl) {
         unsigned char c = (unsigned char)src[i];
         if (c >= 32)
             dest[j++] = src[i];
-        else
+        else if (c > 0)
             strlcatf(dest, sl, j, "%%x%02X", c);
     }
     dest[j++] = '"';