Null: Fix out parameters not being set

It turns out platform functions are sometimes called directly instead of
going through the public wrapper.

src/null_monitor.c

@@ -72,6 +72,10 @@ void _glfwPlatformFreeMonitor(_GLFWmonitor* monitor)
 
 void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
 {
+    if (xpos)
+        *xpos = 0;
+    if (ypos)
+        *ypos = 0;
 }
 
 void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,

src/null_window.c

@@ -269,10 +269,25 @@ void _glfwPlatformGetWindowFrameSize(_GLFWwindow* window,
 {
     if (window->null.decorated && !window->monitor)
     {
-        *left = 1;
-        *top = 10;
-        *right = 1;
-        *bottom = 1;
+        if (left)
+            *left = 1;
+        if (top)
+            *top = 10;
+        if (right)
+            *right = 1;
+        if (bottom)
+            *bottom = 1;
+    }
+    else
+    {
+        if (left)
+            *left = 0;
+        if (top)
+            *top = 0;
+        if (right)
+            *right = 0;
+        if (bottom)
+            *bottom = 0;
     }
 }