fix conflicting options in postgresql service

modules/postgresql.nix

@@ -50,7 +50,7 @@ in
       };
 
       dbConfig = passwordCfgs: {
-        services.postgresql.enable = (builtins.length passwordCfgs) > 0;
+        services.postgresql.enable = lib.mkDefault ((builtins.length passwordCfgs) > 0);
         services.postgresql.ensureDatabases = map ({ database, ... }: database) passwordCfgs;
         services.postgresql.ensureUsers = map ({ username, database, ... }: {
           name = username;

test/modules/postgresql.nix

@@ -36,6 +36,20 @@ in
     expr = testConfig {};
   };
 
+  testPostgresManualOptions = {
+    expected = {
+      services.postgresql = {
+        enable = true;
+        ensureUsers = [];
+        ensureDatabases = [];
+      };
+      systemd.services.postgresql.postStart = "";
+    };
+    expr = testConfig {
+      services.postgresql.enable = true;
+    };
+  };
+
   testPostgresOneWithoutPassword = {
     expected = {
       services.postgresql = {