Skip processing `rule.each` on optional value when input is nil

When a key has a `maybe` macro, nil values are valid. However, if a
`rule.each` is specified on that key, an error is throw saying:

```
`block in each': undefined method `each_with_index' for nil:NilClass (NoMethodError)
```

lib/dry/validation/rule.rb

@@ -82,7 +82,7 @@ module Dry
         @keys = []
 
         @block = proc do
-          unless result.base_error?(root) || !values.key?(root)
+          unless result.base_error?(root) || !values.key?(root) || values[root].nil?
             values[root].each_with_index do |_, idx|
               path = [*Schema::Path[root].to_a, idx]
 

spec/unit/rule_spec.rb

@@ -50,4 +50,24 @@ RSpec.describe Dry::Validation::Contract, ".rule" do
       end
     end
   end
+
+  context "with an optional value" do
+    let(:contract_class) do
+      Class.new(Dry::Validation::Contract) do
+        schema do
+          required(:tags).maybe(:array)
+        end
+      end
+    end
+
+    context "when using .each macro" do
+      it "does not fail when input is nil" do
+        contract_class.rule(:tags).each do
+          key.failure("should not be called")
+        end
+
+        expect(contract.(tags: nil).errors.to_h).to eq({})
+      end
+    end
+  end
 end