Only return 'normal' as the range when no value is given. (#185)

Fixes the error “Can’t assign to property "rangeName": not an object”

Author: bramus

src/proxy-animation.js

@@ -1799,14 +1799,17 @@ function parseTimelineRangeOffset(value, position) {
 
 // Parses a given animation-range value (string)
 function parseAnimationRange(value) {
+  if (!value)
+    return {
+      start: 'normal',
+      end: 'normal'
+    };
+  
   const animationRange = {
-    start: 'normal',
-    end: 'normal'
+    start: { rangeName: 'cover', offset: CSS.percent(0) },
+    end: { rangeName: 'cover', offset: CSS.percent(100) },
   };
 
-  if (!value)
-    return animationRange;
-
   // Format:
   // <start-name> <start-offset> <end-name> <end-offset>
   // <name> --> <name> 0% <name> 100%