[PATCH 1/4] settings: fix -std=c23 build failure

gcc-15 switched to -std=c23 by default:

    https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=55e3bd376b2214e200fa76d12b67ff259b06c212

As a result `ell` fails the build as:

    ell/settings.c: In function 'l_settings_get_embedded_value':
    ell/settings.c:1521:24: error: incompatible types when returning type '_Bool' but 'const char *' was expected
     1521 |                 return false;
          |                        ^~~~~

The change uses poiter instead of a bool to return the zero value.
---
 ell/settings.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ell/settings.c b/ell/settings.c
index a5f17d1..b46d00b 100644
--- a/ell/settings.c
+++ b/ell/settings.c
@@ -1518,7 +1518,7 @@ LIB_EXPORT const char *l_settings_get_embedded_value(
 	struct embedded_group_data *group;
 
 	if (unlikely(!settings))
-		return false;
+		return NULL;
 
 	group = l_queue_find(settings->embedded_groups,
 					embedded_group_match, group_name);
-- 
2.47.0