treewide: Remove usage of isNull

isNull "is deprecated; just write e == null instead" says the Nix manual

nixos/modules/services/databases/cassandra.nix

@@ -22,11 +22,11 @@ let
              else {})
     );
   cassandraConfigWithAddresses = cassandraConfig //
-    ( if isNull cfg.listenAddress
+    ( if cfg.listenAddress == null
         then { listen_interface = cfg.listenInterface; }
         else { listen_address = cfg.listenAddress; }
     ) // (
-      if isNull cfg.rpcAddress
+      if cfg.rpcAddress == null
         then { rpc_interface = cfg.rpcInterface; }
         else { rpc_address = cfg.rpcAddress; }
     );
@@ -219,19 +219,13 @@ in {
   config = mkIf cfg.enable {
     assertions =
       [ { assertion =
-            ((isNull cfg.listenAddress)
-             || (isNull cfg.listenInterface)
-            ) && !((isNull cfg.listenAddress)
-                   && (isNull cfg.listenInterface)
-                  );
+          (cfg.listenAddress == null || cfg.listenInterface == null)
+          && !(cfg.listenAddress == null && cfg.listenInterface == null);
           message = "You have to set either listenAddress or listenInterface";
         }
         { assertion =
-            ((isNull cfg.rpcAddress)
-             || (isNull cfg.rpcInterface)
-            ) && !((isNull cfg.rpcAddress)
-                   && (isNull cfg.rpcInterface)
-                  );
+          (cfg.rpcAddress == null || cfg.rpcInterface == null)
+          && !(cfg.rpcAddress == null && cfg.rpcInterface == null);
           message = "You have to set either rpcAddress or rpcInterface";
         }
       ];
@@ -276,7 +270,7 @@ in {
           };
       };
     systemd.timers.cassandra-full-repair =
-      mkIf (!isNull cfg.fullRepairInterval) {
+      mkIf (cfg.fullRepairInterval != null) {
         description = "Schedule full repairs on Cassandra";
         wantedBy = [ "timers.target" ];
         timerConfig =
@@ -300,7 +294,7 @@ in {
           };
       };
     systemd.timers.cassandra-incremental-repair =
-      mkIf (!isNull cfg.incrementalRepairInterval) {
+      mkIf (cfg.incrementalRepairInterval != null) {
         description = "Schedule incremental repairs on Cassandra";
         wantedBy = [ "timers.target" ];
         timerConfig =

nixos/modules/services/databases/cockroachdb.nix

@@ -7,7 +7,7 @@ let
   crdb = cfg.package;
 
   escape    = builtins.replaceStrings ["%"] ["%%"];
-  ifNotNull = v: s: optionalString (!isNull v) s;
+  ifNotNull = v: s: optionalString (v != null) s;
 
   startupCommand = lib.concatStringsSep " "
     [ # Basic startup
@@ -164,7 +164,7 @@ in
 
   config = mkIf config.services.cockroachdb.enable {
     assertions = [
-      { assertion = !cfg.insecure -> !(isNull cfg.certsDir);
+      { assertion = !cfg.insecure -> cfg.certsDir != null;
         message = "CockroachDB must have a set of SSL certificates (.certsDir), or run in Insecure Mode (.insecure = true)";
       }
     ];

nixos/modules/services/databases/pgmanage.nix

@@ -16,7 +16,7 @@ let
 
       super_only = ${builtins.toJSON cfg.superOnly}
 
-      ${optionalString (!isNull cfg.loginGroup) "login_group = ${cfg.loginGroup}"}
+      ${optionalString (cfg.loginGroup != null) "login_group = ${cfg.loginGroup}"}
 
       login_timeout = ${toString cfg.loginTimeout}
 
@@ -24,7 +24,7 @@ let
 
       sql_root = ${cfg.sqlRoot}
 
-      ${optionalString (!isNull cfg.tls) ''
+      ${optionalString (cfg.tls != null) ''
       tls_cert = ${cfg.tls.cert}
       tls_key = ${cfg.tls.key}
       ''}