Add doc and spec for empty-else node

Follow whitequark/parser#652.

This PR adds doc and spec for `empty-else` node.

Below is a description of `empty-else` node.

> Empty `else` differs from the missing (or _implicit_) `else` for pattern
> matching, since
> the latter one raises a `NoMatchingPattern` exception. Thus, we need a
> way to distinguish this two cases in the resulting AST.

https://github.com/whitequark/parser/blob/v3.0.1.1/doc/AST_FORMAT.md#with-empty-else

Below are the differences between each AST (`empty-else` only appears in the last case).

```console
% ruby-parse -e 'case foo; when bar; end'
(case
  (send nil :foo)
  (when
    (send nil :bar) nil) nil)

% ruby-parse -e 'case foo; when bar; else; end'
(case
  (send nil :foo)
  (when
    (send nil :bar) nil) nil)

% ruby-parse -e 'case foo; in bar; end'
(case-match
  (send nil :foo)
  (in-pattern
    (match-var :bar) nil nil) nil)

% ruby-parse -e 'case foo; in bar; else; end'
(case-match
  (send nil :foo)
  (in-pattern
    (match-var :bar) nil nil)
  (empty-else))
```

This PR has also added a spec for `CaseNode` to clarify the difference in behavior.

lib/rubocop/ast/node/case_match_node.rb

@@ -47,6 +47,7 @@ def branches
       # Returns the else branch of the `case` statement, if any.
       #
       # @return [Node] the else branch node of the `case` statement
+      # @return [EmptyElse] the empty else branch node of the `case` statement
       # @return [nil] if the case statement does not have an else branch.
       def else_branch
         node_parts[-1]

spec/rubocop/ast/case_match_node_spec.rb

@@ -125,6 +125,19 @@
 
           it { expect(case_match_node.else_branch).to be_sym_type }
         end
+
+        context 'with an empty else statement' do
+          let(:source) do
+            <<~RUBY
+              case expr
+              in pattern
+              else
+              end
+            RUBY
+          end
+
+          it { expect(case_match_node.else_branch).to be_empty_else_type }
+        end
       end
     end
 

spec/rubocop/ast/case_node_spec.rb

@@ -104,6 +104,19 @@
 
         it { expect(case_node.else_branch).to be_sym_type }
       end
+
+      context 'with an empty else statement' do
+        let(:source) do
+          <<~RUBY
+            case
+            when :foo then :bar
+            else
+            end
+          RUBY
+        end
+
+        it { expect(case_node.else_branch).to be_nil }
+      end
     end
   end