[FIX] l10n_it_fatturapa_in: _compute method failure
[FIX] l10n_it_fatturapa_in: _compute method failure

This fixes the following errors:
File "/home/travis/build/OCA/l10n-italy/setup/l10n_it_fatturapa_in/odoo/addons/l10n_it_fatturapa_in/models/attachment.py", line 100, in _compute_xml_data
fatt = wiz_obj.get_invoice_obj(att)
File "/home/travis/build/OCA/l10n-italy/setup/l10n_it_fatturapa_in/odoo/addons/l10n_it_fatturapa_in/wizard/wizard_import_fatturapa.py", line 1625, in get_invoice_obj
xml_string = fatturapa_attachment.get_xml_string()
File "/home/travis/build/OCA/l10n-italy/setup/l10n_it_fatturapa_in/odoo/addons/l10n_it_fatturapa_in/models/attachment.py", line 84, in get_xml_string
return self.ir_attachment_id.get_xml_string()
File "/home/travis/build/OCA/l10n-italy/setup/l10n_it_fatturapa/odoo/addons/l10n_it_fatturapa/models/ir_attachment.py", line 76, in get_xml_string
data = base64.b64decode(self.datas)
File "/home/travis/virtualenv/python3.6.7/lib/python3.6/base64.py", line 80, in b64decode
s = _bytes_from_decode_data(s)
File "/home/travis/virtualenv/python3.6.7/lib/python3.6/base64.py", line 46, in _bytes_from_decode_data
"string, not %r" % s.__class__.__name__) from None
TypeError: argument should be a bytes-like object or ASCII string, not 'bool'

The wizard is invoked on an empty model, with no attachment.

OCA/l10n-italy,dcorio/l10n-italy,dcorio/l10n-italy,OCA/l10n-italy,dcorio/l10n-italy,OCA/l10n-italy