Fix mutations on case without conditional

Changelog.md

@@ -2,6 +2,7 @@
 
 Changes:
 
+* Fix crash without conditionals
 * Remove dependency to descendants tracker
 * Add mutation #== => #eql?, #equal?
 * Add mutation #eql? =>  #equal?

lib/mutant/mutator/node/case.rb

@@ -20,7 +20,7 @@ module Mutant
         # @api private
         #
         def dispatch
-          emit_condition_mutations
+          emit_condition_mutations if condition
           emit_when_mutations
           emit_else_mutations
           emit_nil

spec/unit/mutant/mutator/node/case_spec.rb

@@ -9,6 +9,43 @@ describe Mutant::Mutator::Node::Case do
     Mutant::Random.stub(hex_string: random_string)
   end
 
+  context 'without condition' do
+    let(:source) do
+      <<-RUBY
+        case
+        when true
+        else
+        end
+      RUBY
+    end
+
+    let(:mutations) do
+      mutations = []
+      mutations << 'nil'
+      mutations << <<-RUBY
+        case
+        when true
+          raise
+        else
+        end
+      RUBY
+      mutations << <<-RUBY
+        case
+        when false
+        else
+        end
+      RUBY
+      mutations << <<-RUBY
+        case
+        when nil
+        else
+        end
+      RUBY
+    end
+
+    it_should_behave_like 'a mutator'
+  end
+
   context 'with multiple when branches' do
     let(:source) do
       <<-RUBY