[FIX] Don't break printing some upstream reports with invalid models

When no background configuration is present on the report definition,
don't try to browse the report model especially because this breaks
some upstream reports (see odoo/odoo#23389)

report_qweb_pdf_watermark/__manifest__.py

@@ -3,7 +3,7 @@
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 {
     "name": "Pdf watermark",
-    "version": "10.0.1.0.1",
+    "version": "10.0.1.0.2",
     "author": "Therp BV, "
               "Odoo Community Association (OCA)",
     "license": "AGPL-3",

report_qweb_pdf_watermark/models/report.py

@@ -34,9 +34,9 @@ def get_pdf(self, docids, report_name, html=None, data=None):
         watermark = None
         if report.pdf_watermark:
             watermark = b64decode(report.pdf_watermark)
-        else:
+        elif report.pdf_watermark_expression:
             watermark = tools.safe_eval(
-                report.pdf_watermark_expression or 'None',
+                report.pdf_watermark_expression,
                 dict(env=self.env, docs=self.env[report.model].browse(docids)),
             )
             if watermark: