fix: add presence validation for account name (#12636)

## Description

When a user tries creating a new account through the Super Admin
dashboard, and they forget to fill in the account name, they're faced
with an ugly error (generic "Something went wrong" on production).

This PR simply adds the `validates :name, presence: true` model
validation on `Account` model, which is translated as a proper error
message on the Super Admin UI.

spec/models/account_spec.rb

@@ -22,6 +22,12 @@ RSpec.describe Account do
   describe 'length validations' do
     let(:account) { create(:account) }
 
+    it 'validates name presence' do
+      account.name = ''
+      account.valid?
+      expect(account.errors[:name]).to include("can't be blank")
+    end
+
     it 'validates name length' do
       account.name = 'a' * 256
       account.valid?