add lib.strings.conditionalString

nix/lib/default.nix

@@ -13,6 +13,7 @@ nixpkgs.lib
   math = ./math.nix;
   modules = ./modules.nix;
   network = ./network.nix;
+  strings = ./strings.nix;
   types = ./types.nix;
   x-banananetwork-unused = ./unused.nix;
 }

nix/lib/strings.nix

@@ -0,0 +1,32 @@
+{ lib, ... }@flakeArg:
+let
+  inherit (builtins)
+    isAttrs
+    isBool
+    isList
+    isNull
+    isString
+    typeOf
+    ;
+  inherit (lib.strings) optionalString;
+in
+{
+
+  conditionalString =
+    cond:
+    optionalString (
+      if isNull cond then
+        false
+      else if isBool cond then
+        cond
+      else if isString cond then
+        cond != ""
+      else if isList cond then
+        cond != [ ]
+      else if isAttrs cond then
+        cond.enable or (cond != { })
+      else
+        throw "unexpected type of condition ${typeOf cond}"
+    );
+
+}