Make Left the default "side" for toggleDrawer (#2921)

Avoids a null pointer exception when not passing { side: "left" } to navigator.toggleDrawer() on Android

android/app/src/main/java/com/reactnativenavigation/views/SideMenu.java

@@ -30,7 +30,7 @@ public class SideMenu extends DrawerLayout {
         }
 
         public static Side fromString(String side) {
-            return "left".equals(side.toLowerCase()) ? Left : Right;
+            return (side == null) || "left".equals(side.toLowerCase()) ? Left : Right;
         }
     }
 
@@ -181,4 +181,4 @@ public class SideMenu extends DrawerLayout {
             setDrawerLockMode(DrawerLayout.LOCK_MODE_LOCKED_CLOSED, params.side.gravity);
         }
     }
-}
+}