[FIX] tools: better support 'almost empty' content from editor

It seems quite easy to have a void content in the currently new editor
that actually holds a lot of undesired information, notably a font
tag. This is annoying when having behavior based on an html field
being empty or not. In this commit we therefore improve definition
of an 'empty' html field.

Task ID-2532529
PR odoo/odoo#71793

odoo/addons/base/tests/test_mail.py

@@ -328,13 +328,21 @@ class TestHtmlTools(BaseCase):
         for content in void_strings_samples:
             self.assertTrue(is_html_empty(content))
 
-        void_html_samples = ['<p><br></p>', '<p><br> </p>', '<p><br /></p >', '<p style="margin: 4px"></p>',
-                             '<div style="margin: 4px"></div>', '<p class="oe_testing"><br></p>']
+        void_html_samples = [
+            '<p><br></p>', '<p><br> </p>', '<p><br /></p >',
+            '<p style="margin: 4px"></p>',
+            '<div style="margin: 4px"></div>',
+            '<p class="oe_testing"><br></p>',
+            '<p><span style="font-weight: bolder;"><font style="color: rgb(255, 0, 0);" class=" "></font></span><br></p>',
+        ]
         for content in void_html_samples:
             self.assertTrue(is_html_empty(content), 'Failed with %s' % content)
 
-        valid_html_samples = ['<p><br>1</p>', '<p>1<br > </p>', '<p style="margin: 4px">Hello World</p>',
-                              '<div style="margin: 4px"><p>Hello World</p></div>']
+        valid_html_samples = [
+            '<p><br>1</p>', '<p>1<br > </p>', '<p style="margin: 4px">Hello World</p>',
+            '<div style="margin: 4px"><p>Hello World</p></div>',
+            '<p><span style="font-weight: bolder;"><font style="color: rgb(255, 0, 0);" class=" ">W</font></span><br></p>',
+        ]
         for content in valid_html_samples:
             self.assertFalse(is_html_empty(content))
 

odoo/tools/mail.py

@@ -280,7 +280,7 @@ def is_html_empty(html_content):
     """
     if not html_content:
         return True
-    tag_re = re.compile(r'\<\s*\/?(?:p|div|span|br|b|i)(?:(?=\s+\w*)[^/>]*|\s*)/?\s*\>')
+    tag_re = re.compile(r'\<\s*\/?(?:p|div|span|br|b|i|font)(?:(?=\s+\w*)[^/>]*|\s*)/?\s*\>')
     return not bool(re.sub(tag_re, '', html_content).strip())