lambda(&:nil?) in specs is replaced with explicit lambdas

Rails 7.0 evaluates :if/:unless callback conditions via
instance_exec, which doesn't pass the instance as an argument
to Symbol#to_proc lambdas, causing "no receiver given" errors.

Author: AlfonsoUceda

spec/validations/validates_db_uniqueness_of_spec.rb

@@ -233,7 +233,7 @@ def catch_error_message
             context 'when proc returns false' do
               let(:klass) do
                 define_class do
-                  validates_db_uniqueness_of :field, if: lambda(&:nil?)
+                  validates_db_uniqueness_of :field, if: ->(entity) { entity.nil? } # rubocop:disable Style/SymbolProc
                 end
               end
 
@@ -315,7 +315,7 @@ def catch_error_message
             context 'when proc returns false' do
               let(:klass) do
                 define_class do
-                  validates_db_uniqueness_of :field, unless: lambda(&:nil?)
+                  validates_db_uniqueness_of :field, unless: ->(entity) { entity.nil? } # rubocop:disable Style/SymbolProc
                 end
               end