fix: pass empty string email content of pos invoice

[mmdanny89]

When complete order in point of sale (Order Summary). When we send invoice email frappe send this error:

TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Traceback (most recent call last):
File "apps/frappe/frappe/app.py", line 95, in application
response = frappe.api.handle()
File "apps/frappe/frappe/api.py", line 54, in handle
return frappe.handler.handle()
File "apps/frappe/frappe/handler.py", line 47, in handle
data = execute_cmd(cmd)
File "apps/frappe/frappe/handler.py", line 85, in execute_cmd
return frappe.call(method, **frappe.form_dict)
File "apps/frappe/frappe/init.py", line 1622, in call
return fn(*args, **newargs)
File "apps/frappe/frappe/core/doctype/communication/email.py", line 79, in make
return _make(
File "apps/frappe/frappe/core/doctype/communication/email.py", line 173, in _make
comm.send_email(
File "apps/frappe/frappe/core/doctype/communication/mixins.py", line 305, in send_email
if input_dict := self.sendmail_input_dict(
File "apps/frappe/frappe/core/doctype/communication/mixins.py", line 284, in sendmail_input_dict
"content": self.get_content(print_format=print_format),
File "apps/frappe/frappe/core/doctype/communication/mixins.py", line 142, in get_content
return self.content + self.get_attach_link(print_format)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Request Data

{
"type": "POST",
"args": {
"recipients": "mmdanny89@gmail.com",
"subject": "POS Invoice: ACC-PSINV-2024-00035",
"doctype": "POS Invoice",
"name": "ACC-PSINV-2024-00035",
"send_email": 1,
"print_format": "Invoice Print POS",
"sender_full_name": "Danny Molina",
"_lang": "es"
},
"headers": {},
"error_handlers": {},
"url": "/api/method/frappe.core.doctype.communication.email.make"
}

In view i added empty content for this email.make call this slove the error

[frappe-pr-bot]

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