fix(test): _NativeSafeAreaContext.default.getConstants is not a funct…

…ion (#335)

Fixes #290 
Fixes #255 

Verified to work in our own projects. The reason is described in #290, it appears that the native module import comes back as `{ default: {} }`. So `default.getConstants` is `undefined`. The fix ensures it's only called if defined.

src/InitialWindow.native.ts

@@ -1,7 +1,7 @@
 import type { Metrics } from './SafeArea.types';
 import NativeSafeAreaContext from './specs/NativeSafeAreaContext';
 
-export const initialWindowMetrics = (NativeSafeAreaContext?.getConstants()
+export const initialWindowMetrics = (NativeSafeAreaContext?.getConstants?.()
   ?.initialWindowMetrics ?? null) as Metrics | null;
 
 /**