feat: switch from rage to age

Why
===
* Someone said age works better with password protected keys,
requiring entering the password less often.
* We switched to rage from age in
07ce686870
because it was limiting recipients to 20. This was fixed
https://github.com/FiloSottile/age/issues/139

What changed
===
* Switch from rage back to age (the reference implementation) in all
the spots
* Update the docs to show how to switch back to Rage
* Skip keys that are empty files, which fixes the integration test.

modules/age-home.nix

@@ -155,7 +155,7 @@ with lib; let
     '';
 in {
   options.age = {
-    package = mkPackageOption pkgs "rage" {};
+    package = mkPackageOption pkgs "age" {};
 
     secrets = mkOption {
       type = types.attrsOf secretType;

modules/age.nix

@@ -69,6 +69,7 @@ with lib; let
     IDENTITIES=()
     for identity in ${toString cfg.identityPaths}; do
       test -r "$identity" || continue
+      test -s "$identity" || continue
       IDENTITIES+=(-i)
       IDENTITIES+=("$identity")
     done
@@ -189,9 +190,9 @@ in {
   options.age = {
     ageBin = mkOption {
       type = types.str;
-      default = "${pkgs.rage}/bin/rage";
+      default = "${pkgs.age}/bin/age";
       defaultText = literalExpression ''
-        "''${pkgs.rage}/bin/rage"
+        "''${pkgs.age}/bin/age"
       '';
       description = ''
         The age executable to use.