nixos/wireless: create empty config for imperative setup

If allowAuxiliaryImperativeNetworks is enabled, the wpa_supplicant
daemon complains fails to start if /etc/wpa_supplicant.conf does not
exist. As this can be created using wpa_cli (or similar) later, it
shouldn't matter, so let's create an empty one.

This fixes issues #157537, #299466.

nixos/modules/services/networking/wpa_supplicant.nix

@@ -124,6 +124,11 @@ let
           fi
         ''}
 
+        # ensure wpa_supplicant.conf exists, or the daemon will fail to start
+        ${optionalString cfg.allowAuxiliaryImperativeNetworks ''
+          touch /etc/wpa_supplicant.conf
+        ''}
+
         # substitute environment variables
         if [ -f "${configFile}" ]; then
           ${pkgs.gawk}/bin/awk '{