[FIX] account: prevent error will uploading empty text attachment in invoice

When the user uploads an empty text file in the invoice it generates an
attribute error with 'bool' object getting no attribute 'startswith'.

Steps to produce:-
- Install 'account'
- Go to 'Invoice'
- Create an invoice with an empty .txt attachment in the chatter section
- Click 'Send'
- Traceback is generated

Applying this commit will fix this issue.

sentry-4187919992

closes odoo/odoo#122103

X-original-commit: 7a12da836fa9926e12b15484aad77224cb75d3a3
Signed-off-by: William André (wan) <wan@odoo.com>

addons/account/models/ir_attachment.py

@@ -100,7 +100,7 @@ class IrAttachment(models.Model):
             # XML attachments received by mail have a 'text/plain' mimetype (cfr. context key:
             # 'attachments_mime_plainxml'). Therefore, if content start with '<?xml', or if the filename ends with
             # '.xml', it is considered as XML.
-            is_text_plain_xml = 'text/plain' in attachment.mimetype and (attachment.raw.startswith(b'<?xml') or attachment.name.endswith('.xml'))
+            is_text_plain_xml = 'text/plain' in attachment.mimetype and (attachment.raw and attachment.raw.startswith(b'<?xml') or attachment.name.endswith('.xml'))
             return attachment.mimetype.endswith('/xml') or is_text_plain_xml
 
         return [