Fix thumbnail_img NoneType error (#2941)

### What problem does this PR solve?

fix thumbnail_img NoneType error

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/utils/file_utils.py

@@ -199,8 +199,11 @@ def thumbnail_img(filename, blob):
 
 def thumbnail(filename, blob):
     img = thumbnail_img(filename, blob)
-    return IMG_BASE64_PREFIX + \
-        base64.b64encode(img).decode("utf-8")
+    if img is not None:
+        return IMG_BASE64_PREFIX + \
+            base64.b64encode(img).decode("utf-8")
+    else:
+        return ''
 
 def traversal_files(base):
     for root, ds, fs in os.walk(base):