all: drop str_if_set() in favor of the "?:" operator

str_if_set() was added to replace the non-standard gcc extension "?:".
However, "?:" is supported by clang as well and we already use it at
several places.

Also, str_if_set() did not follow our naming scheme and renaming to
nm_str_if_set() would be ugly. So just drop it.

src/platform/nm-linux-platform.c

@@ -4018,7 +4018,7 @@ link_get_type_name (NMPlatform *platform, int ifindex)
 		return nm_link_type_to_string (obj->link.type);
 	}
 	/* Link type not detected. Fallback to rtnl_link_get_type()/IFLA_INFO_KIND. */
-	return str_if_set (obj->link.kind, "unknown");
+	return obj->link.kind ?: "unknown";
 }
 
 static gboolean