Pass NULL instead of empty string to option parser

A bug, introduced in #576. If alignment isn't set in the config, the
settings would print a warning about an invalid value. But actually it's
just the default string, which can't get parsed.

The fallback value should get handled by a false return value and not by
the option_get_string's default value.

src/settings.c

@@ -277,7 +277,7 @@ void load_settings(char *cmdline_config_path)
         {
                 char *c = option_get_string(
                         "global",
-                        "alignment", "-align/-alignment", "",
+                        "alignment", "-align/-alignment", NULL,
                         "Text alignment left/center/right"
                 );