Allow empty keyframe and keyframes with non-animatable properties.

We need to create CSS animations that have empty keyframe or keyframes
which have only invalid properties or non-animatable properties to fire
animation events for such animations.

tests/unit/style/keyframes.rs

@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use parking_lot::RwLock;
+use std::sync::Arc;
+use style::keyframes::{Keyframe, KeyframesAnimation, KeyframePercentage,  KeyframeSelector};
+use style::properties::PropertyDeclarationBlock;
+
+#[test]
+fn test_empty_keyframe() {
+    let keyframes = vec![];
+    let animation = KeyframesAnimation::from_keyframes(&keyframes);
+    let expected = KeyframesAnimation {
+        steps: vec![],
+        properties_changed: vec![],
+    };
+
+    assert_eq!(format!("{:#?}", animation), format!("{:#?}", expected));
+}
+
+#[test]
+fn test_no_property_in_keyframe() {
+    let keyframes = vec![
+        Arc::new(RwLock::new(Keyframe {
+            selector: KeyframeSelector::new_for_unit_testing(vec![KeyframePercentage::new(1.)]),
+            block: Arc::new(RwLock::new(PropertyDeclarationBlock {
+                declarations: vec![],
+                important_count: 0,
+            }))
+        })),
+    ];
+    let animation = KeyframesAnimation::from_keyframes(&keyframes);
+    let expected = KeyframesAnimation {
+        steps: vec![],
+        properties_changed: vec![],
+    };
+
+    assert_eq!(format!("{:#?}", animation), format!("{:#?}", expected));
+}