[iOS] Protect animation argument missing which cause crash on iOS. (#2771)

Author: wqyfavor

ios/sdk/WeexSDK/Sources/Module/WXAnimationModule.m

@@ -332,6 +332,10 @@ - (void)transitionWithArgs:(NSDictionary *)args withProperty:(NSString *)propert
                 transition:(WXTransition*)transition
              transitionDic:(NSMutableDictionary*)transitionDic
 {
+    if (args[@"styles"][property] == nil) {
+        return;
+    }
+    
     [transition.filterStyles setObject:args[@"styles"][property] forKey:property];
 
     id oldStyleValue = target.styles[property];
@@ -346,7 +350,7 @@ - (void)transitionWithArgs:(NSDictionary *)args withProperty:(NSString *)propert
     [target _modifyStyles:@{property:args[@"styles"][property]}];
     [transitionDic setObject:@([args[@"duration"] doubleValue]) forKey:kWXTransitionDuration];
     [transitionDic setObject:@([args[@"delay"] doubleValue]) forKey:kWXTransitionDelay];
-    [transitionDic setObject:args[@"timingFunction"] forKey:kWXTransitionTimingFunction];
+    [transitionDic setObject:args[@"timingFunction"] ?: @"linear" forKey:kWXTransitionTimingFunction];
 }
 
 - (void)animation:(WXComponent *)targetComponent args:(NSDictionary *)args callback:(WXModuleKeepAliveCallback)callback