FIX: return nil correctly for nil check (#37460)

## 🔍 Overview

This update is a follow-up to:
https://github.com/discourse/discourse/pull/36813. The PR had a
follow-up adding `nil` checks to the `DeleteUserPosts` job, but since
the look-ups were using `find` instead of `find_by` a `RecordNotFound`
error would be returned instead of `nil`. This update ensures that we
use `find_by` instead so that the `nil` checks work correctly.
Additionally, we add some tests to ensure the job isn't triggered when
the params are missing.

app/jobs/regular/delete_user_posts.rb

@@ -5,10 +5,10 @@ module Jobs
     sidekiq_options queue: "critical"
 
     def execute(args)
-      user = User.find(args[:user_id])
+      user = User.find_by(id: args[:user_id])
       return if user.nil?
 
-      acting_user = User.find(args[:acting_user_id]) if args[:acting_user_id]
+      acting_user = User.find_by(id: args[:acting_user_id])
       return if acting_user.nil?
 
       guardian = Guardian.new(acting_user)

spec/jobs/delete_user_posts_spec.rb

@@ -48,4 +48,16 @@ RSpec.describe Jobs::DeleteUserPosts do
     user.reload
     expect(user.posts.count).to eq(0)
   end
+
+  it "does nothing if user does not exist" do
+    expect {
+      described_class.new.execute(user_id: -999, acting_user_id: admin.id)
+    }.not_to raise_error
+  end
+
+  it "does nothing if acting_user does not exist" do
+    expect {
+      described_class.new.execute(user_id: user.id, acting_user_id: -999)
+    }.not_to raise_error
+  end
 end