config: inject TSDBConfig defaults in Load for empty config bodies

When the config body is empty, UnmarshalYAML is never called, so the
TSDBConfig nil injection added there never ran. Replicate the same guard
in Load, which is the entry point that already handles this case for
other defaults via DefaultConfig.

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>

config/config.go

@@ -83,6 +83,13 @@ func Load(s string, logger *slog.Logger) (*Config, error) {
 		return nil, err
 	}
 
+	// When the config body is empty, UnmarshalYAML is never called, so
+	// TSDBConfig may still be nil.
+	if cfg.StorageConfig.TSDBConfig == nil {
+		retention := DefaultTSDBRetentionConfig
+		cfg.StorageConfig.TSDBConfig = &TSDBConfig{Retention: &retention}
+	}
+
 	b := labels.NewScratchBuilder(0)
 	cfg.GlobalConfig.ExternalLabels.Range(func(v labels.Label) {
 		newV := os.Expand(v.Value, func(s string) string {

config/config_test.go

@@ -2665,6 +2665,8 @@ func TestEmptyConfig(t *testing.T) {
 	require.NoError(t, err)
 	exp := DefaultConfig
 	exp.loaded = true
+	retention := DefaultTSDBRetentionConfig
+	exp.StorageConfig.TSDBConfig = &TSDBConfig{Retention: &retention}
 	require.Equal(t, exp, *c)
 	require.Equal(t, 75, c.Runtime.GoGC)
 }