Don’t crash when inherited SVG attributes are not set on the parent

Author: liZe

tests/draw/svg/test_shapes.py

@@ -435,3 +435,26 @@ def test_rect_width_height_zero(assert_pixels):
             <rect x="2" y="2" width="5" height="5" fill="red" />
         </svg>
     ''')
+
+
+@assert_no_logs
+def test_rect_fill_inherit(assert_pixels):
+    assert_pixels('''
+        _________
+        _________
+        __KKKKK__
+        __KKKKK__
+        __KKKKK__
+        __KKKKK__
+        __KKKKK__
+        _________
+        _________
+    ''', '''
+      <style>
+        @page { size: 9px }
+        svg { display: block }
+      </style>
+      <svg width="9px" height="9px" xmlns="http://www.w3.org/2000/svg">
+        <rect x="2" y="2" width="5" height="5" fill="inherit" />
+      </svg>
+    ''')

weasyprint/svg/__init__.py

@@ -150,9 +150,13 @@ def __iter__(self):
                     child.attrib[key] = child.get('color', 'black')
 
             # Handle 'inherit' values
-            for key, value in child.attrib.items():
+            for key, value in child.attrib.copy().items():
                 if value == 'inherit':
-                    child.attrib[key] = self.get(key)
+                    value = self.get(key)
+                    if value is None:
+                        del child.attrib[key]
+                    else:
+                        child.attrib[key] = value
 
             # Fix text in text tags
             if child.tag in ('text', 'textPath', 'a'):