[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 https://github.com/odoo/odoo/pull/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 @@ class Report(models.Model):
         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: