Fix index out of range when calculating tag intersection

Shouldn't rely on "matched tags" returning any results, if the tag
fails to find anything we shouldn't continue to find the intersection
of the non-existant tags.

notebook/notebook.go

@@ -304,6 +304,11 @@ func (book *Notebook) MatchedTags(searchTags ...string) []matchedTag {
 
 func (book *Notebook) TagIntersection(matchedTags []matchedTag) []graph.Vertex {
 	var intersection = make(map[uint64]graph.Vertex)
+
+	if len(matchedTags) == 0 {
+		return nil
+	}
+
 	var mostConnectedVertex = matchedTags[0]
 VERTEX:
 	for vertexId := range book.Notes.Adjacency[mostConnectedVertex.Vertex.Id] {