Make joystick platform code init on demand

This makes joystick support initialize the first time a joystick
function is called, including those gamepad functions that are layered
on top of joystick functions.

Related to #1284.
Related to #1646.

src/null_joystick.c

@@ -33,6 +33,15 @@
 //////                       GLFW platform API                      //////
 //////////////////////////////////////////////////////////////////////////
 
+int _glfwPlatformInitJoysticks(void)
+{
+    return GLFW_TRUE;
+}
+
+void _glfwPlatformTerminateJoysticks(void)
+{
+}
+
 int _glfwPlatformPollJoystick(_GLFWjoystick* js, int mode)
 {
     return GLFW_FALSE;