Make formatter not strict in all systems and try to...

... avoid broken attributes in cases where we can predict that
we don't have any.

This is far from ideal, but it's the best we can do.
If it becomes a shit show, flake authors can opt out.

dev/tests/nix-unit.nix

@@ -464,4 +464,68 @@ in
       expected = "dogfood";
     };
   };
+
+  formatter = {
+    "test: conditional null throws helpful error" = {
+      expr =
+        let
+          result = mkFlake { inputs.self = { }; } {
+            systems = [ "x86_64-linux" "aarch64-darwin" ];
+            perSystem = { system, ... }: {
+              formatter =
+                if system == "x86_64-linux" then
+                  derivation { name = "fmt"; builder = "x"; system = system; }
+                else null;
+            };
+          };
+        in
+        result.formatter.aarch64-darwin;
+      expectedError.type = "ThrownError";
+      expectedError.msg = "could not determine statically(.|\n)*disabledModules(.|\n)*inputs\\.flake-parts\\.modules\\.formatter";
+    };
+
+    "test: empty when never defined" = {
+      expr =
+        let
+          result = mkFlake { inputs.self = { }; } {
+            systems = [ "x86_64-linux" "aarch64-darwin" ];
+            perSystem = { ... }: { };
+          };
+        in
+        result.formatter == { };
+      expected = true;
+    };
+
+    "test: present for all systems" = {
+      expr =
+        let
+          result = mkFlake { inputs.self = { }; } {
+            systems = [ "x86_64-linux" "aarch64-darwin" ];
+            perSystem = { system, ... }: {
+              formatter = derivation { name = "fmt"; builder = "x"; inherit system; };
+            };
+          };
+        in
+        result ? formatter && result.formatter ? x86_64-linux && result.formatter ? aarch64-darwin;
+      expected = true;
+    };
+
+    "test: lazy per-system" = {
+      expr =
+        let
+          result = mkFlake { inputs.self = { }; } {
+            systems = [ "x86_64-linux" "aarch64-darwin" ];
+            perSystem = { system, ... }: {
+              formatter =
+                if system == "x86_64-linux" then
+                  derivation { name = "fmt"; builder = "x"; system = system; }
+                else
+                  throw "should not evaluate aarch64-darwin formatter";
+            };
+          };
+        in
+        result.formatter.x86_64-linux.name;
+      expected = "fmt";
+    };
+  };
 }