Fix `rake routes` error when `Rails::Engine` with empty routes is mounted; fixes rails/rails#13810

Squash
Merge pull request #13811 from mdesantis/issue-13810

Fixes #13810 `rake routes` error when mount `Rails::Engine` with empty routes
app_rails_loader.rb should check if bin/rails is a File before calling File.read(exe); closes #13825
Merge pull request #13828 from bronzle/apploader-should-check-for-file

app_rails_loader.rb should check if bin/rails is a File before calling F...
Add a test-case for GH #13825
Reordered classes in AR Validation #validates_with example [ci skip]

Person called GoodnessValidator before it was defined.
This change will compile the example correctly.
Merge pull request #13829 from qsymmachus/improve_validates_with_example

Reordered classes in AR Validation #validates_with example [ci skip]
Fix `ActiveRecord::RecordNotFound` error message with custom primary key
Merge pull request #13785 from kuldeepaggarwal/fix-find_with_multiple_ids

Fix `ActiveRecord::RecordNotFound` error message with custom primary key
Remove unused argument.
Rewrote the tests for has_secure_password
Restored the ability to clear the password with user.password= nil (see the docs)
Got all the new tests passing
Removed old tests
Some minor fixes
Merge pull request #13835 from frodsan/remove_unused_argument

Remove unused argument.
Merge pull request #13772 from chancancode/fix_has_secure_password_for_good

Refactored validations rules for has_secure_password