Add handling of undefined cases for tsc

The handled cases are never going to actually be undefined,
as the affected variables are defined before.
tsc however was not satisfied so these changed had to be made.

src/utils/jellyfin-apiclient/getItems.ts

@@ -42,9 +42,9 @@ function mergeResults(results: BaseItemDtoQueryResult[]) {
             console.log('[getItems] Retrieved StartIndex is invalid', result.StartIndex);
             continue;
         }
-        merged.Items = merged.Items.concat(result.Items);
-        merged.TotalRecordCount += result.TotalRecordCount;
-        merged.StartIndex = Math.min(merged.StartIndex, result.StartIndex);
+        merged.Items = merged.Items?.concat(result.Items);
+        merged.TotalRecordCount! += result.TotalRecordCount;
+        merged.StartIndex = Math.min(merged.StartIndex || 0, result.StartIndex);
     }
     return merged;
 }