Cocoa: Remove calls to -update on nil

These calls have no effect unless the context was created with NSGL.

src/cocoa_window.m

@@ -243,7 +243,7 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
 
 - (void)windowDidResize:(NSNotification *)notification
 {
-    if (window->context.client != GLFW_NO_API)
+    if (window->context.source == GLFW_NATIVE_CONTEXT_API)
         [window->context.nsgl.object update];
 
     if (_glfw.ns.disabledCursorWindow == window)
@@ -278,7 +278,7 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
 
 - (void)windowDidMove:(NSNotification *)notification
 {
-    if (window->context.client != GLFW_NO_API)
+    if (window->context.source == GLFW_NATIVE_CONTEXT_API)
         [window->context.nsgl.object update];
 
     if (_glfw.ns.disabledCursorWindow == window)
@@ -397,7 +397,7 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
 
 - (void)updateLayer
 {
-    if (window->context.client != GLFW_NO_API)
+    if (window->context.source == GLFW_NATIVE_CONTEXT_API)
         [window->context.nsgl.object update];
 
     _glfwInputWindowDamage(window);