* 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.