Fix a nil pointer dereference in GetSourceNames() (#541)

Fix a nil pointer dereference in `GetSourceNames()`, which happened
quite often for me while developing Quesma - I often start Quesma before
Elastic is fully up, therefore `im.sources` would be nil (failed to load
from Elastic) and `GetSourceNames()` would panic.

quesma/elasticsearch/index_management.go

@@ -50,12 +50,17 @@ func (im *indexManagement) ReloadIndices() {
 }
 
 func (im *indexManagement) GetSources() Sources {
-	return *im.sources.Load()
+	if s := im.sources.Load(); s != nil {
+		return *s
+	} else {
+		logger.Warn().Msg("Indices are not yet loaded, returning empty sources.")
+		return Sources{}
+	}
 }
 
 func (im *indexManagement) GetSourceNames() map[string]bool {
 	names := make(map[string]bool)
-	sources := *im.sources.Load()
+	sources := im.GetSources()
 	for _, stream := range sources.DataStreams {
 		names[stream.Name] = true
 	}