FIX: base import script was not updating first_post_created_at column

FEATURE: new rake task to update first_post_created_at column

The not-equal operator (`<>`) in PostgreSQL does not compare values
with NULL. We should instead use `IS DISTINCT FROM` when comparing
values with NULL.

lib/tasks/import.rake

@@ -497,3 +497,22 @@ task 'import:file', [:file_name] => [:environment] do |_, args|
   ImportExport.import(args[:file_name])
   puts "", "Done", ""
 end
+
+desc "Update first_post_created_at column in user_stats table"
+task "import:update_first_post_created_at" => :environment do
+  puts "", "Updating first_post_created_at..."
+
+  DB.exec <<~SQL
+    WITH sub AS (
+      SELECT user_id, MIN(posts.created_at) AS first_post_created_at
+      FROM posts
+      GROUP BY user_id
+    )
+    UPDATE user_stats
+    SET first_post_created_at = sub.first_post_created_at
+    FROM user_stats u1
+    JOIN sub ON sub.user_id = u1.user_id
+    WHERE u1.user_id = user_stats.user_id
+      AND user_stats.first_post_created_at IS DISTINCT FROM sub.first_post_created_at
+  SQL
+end

script/import_scripts/base.rb

@@ -684,7 +684,7 @@ class ImportScripts::Base
       FROM users u1
       JOIN lpa ON lpa.user_id = u1.id
       WHERE u1.id = users.id
-        AND users.last_posted_at <> lpa.last_posted_at
+        AND users.last_posted_at IS DISTINCT FROM lpa.last_posted_at
     SQL
   end
 
@@ -702,7 +702,7 @@ class ImportScripts::Base
       FROM user_stats u1
       JOIN sub ON sub.user_id = u1.user_id
       WHERE u1.user_id = user_stats.user_id
-        AND user_stats.first_post_created_at <> sub.first_post_created_at
+        AND user_stats.first_post_created_at IS DISTINCT FROM sub.first_post_created_at
     SQL
 
     puts "", "Updating user post_count..."