Handle glfwGetPrimaryMonitor returning nullptr (#20523)

* Handle glfwGetPrimaryMonitor returning nullptr

[glfwGetPrimaryMonitor](https://www.glfw.org/docs/3.3/group__monitor.html#ga721867d84c6d18d6790d64d2847ca0b1) may return NULL in non-error cases when no monitor is found. This started to happen recently with an update to the linux distribution, see internal b/162545223.

Handling this case avoids a crash and instead assumes a pixel density.

shell/platform/glfw/flutter_glfw.cc

@@ -184,6 +184,9 @@ static FlutterDesktopMessage ConvertToDesktopMessage(
 // that a screen coordinate is one dp.
 static double GetScreenCoordinatesPerInch() {
   auto* primary_monitor = glfwGetPrimaryMonitor();
+  if (primary_monitor == nullptr) {
+    return kDpPerInch;
+  }
   auto* primary_monitor_mode = glfwGetVideoMode(primary_monitor);
   int primary_monitor_width_mm;
   glfwGetMonitorPhysicalSize(primary_monitor, &primary_monitor_width_mm,