Don’t crash with empty CSS values

Fix #2012.

tests/test_css_validation.py

@@ -8,6 +8,8 @@
 from weasyprint.css import preprocess_declarations
 from weasyprint.css.computed_values import ZERO_PIXELS
 from weasyprint.css.properties import INITIAL_VALUES
+from weasyprint.css.validation.expanders import EXPANDERS
+from weasyprint.css.validation.properties import PROPERTIES
 from weasyprint.images import LinearGradient, RadialGradient
 
 from .testing_utils import assert_no_logs, capture_logs
@@ -1248,3 +1250,10 @@ def test_image_orientation(rule, result):
 ))
 def test_image_orientation_invalid(rule, reason):
     assert_invalid(rule, reason)
+
+
+@assert_no_logs
+#@pytest.mark.parametrize('expander', list(EXPANDERS) + list(PROPERTIES))
+@pytest.mark.parametrize('expander', list(EXPANDERS))
+def test_empty_value(expander):
+    assert_invalid(f'{expander}:', message='Ignored')

weasyprint/css/validation/__init__.py

@@ -169,6 +169,10 @@ def validation_error(level, reason):
         expander_ = EXPANDERS.get(name, validate_non_shorthand)
         tokens = remove_whitespace(declaration.value)
         try:
+            # Having no tokens is allowed by grammar but refused by all
+            # properties and expanders.
+            if not tokens:
+                raise InvalidValues('no value')
             # Use list() to consume generators now and catch any error.
             result = list(expander_(name, tokens, base_url))
         except InvalidValues as exc: