fix: median function fails if it receives an empty table (#316)

* _median function fails if receive empty table.

* fix: median for 1 datapoint

Co-authored-by: Jérôme Wiedemann <jerome@wnetworks.org>

src/graphEntry.ts

@@ -513,6 +513,8 @@ export default class GraphEntry {
   private _median(items: EntityCachePoints) {
     // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
     const itemsDup = this._filterNulls([...items]).sort((a, b) => a[1]! - b[1]!);
+    if (itemsDup.length === 0) return null;
+    if (itemsDup.length === 1) return itemsDup[0][1];
     const mid = Math.floor((itemsDup.length - 1) / 2);
     if (itemsDup.length % 2 === 1) return itemsDup[mid][1];
     // eslint-disable-next-line @typescript-eslint/no-non-null-assertion