server: Fix regression in uploading images

The attachment is first saved with no content while the image is being
asynchronously resized. On our side we had a guard condition fail if the
content was empty, whereas the original implementation was simply using
an empty string instead.

src/becca/entities/bnote.ts

@@ -1644,10 +1644,7 @@ class BNote extends AbstractBeccaEntity<BNote> {
             position
         });
 
-        if (!content) {
+        content = content || "";
-            throw new Error("Attempted to save an attachment with no content.");
-        }
-
         attachment.setContent(content, {forceSave: true});
 
         return attachment;