Remove window requirement for all event functions

This lets an application wait for non-window events without needing to
create a window.

Fixes #1317.

src/window.c

@@ -1078,10 +1078,6 @@ GLFWAPI void glfwPollEvents(void)
 GLFWAPI void glfwWaitEvents(void)
 {
     _GLFW_REQUIRE_INIT();
-
-    if (!_glfw.windowListHead)
-        return;
-
     _glfwPlatformWaitEvents();
 }
 
@@ -1104,10 +1100,6 @@ GLFWAPI void glfwWaitEventsTimeout(double timeout)
 GLFWAPI void glfwPostEmptyEvent(void)
 {
     _GLFW_REQUIRE_INIT();
-
-    if (!_glfw.windowListHead)
-        return;
-
     _glfwPlatformPostEmptyEvent();
 }