Move nil body mutation to parent nodes

This ensures mutant all the time creates semantically valid asts.

lib/mutant/mutator/node/begin.rb

@@ -24,7 +24,6 @@ module Mutant
           children.each do |child|
             emit(child)
           end
-          emit(nil) unless parent_send?
         end
 
         # Test if parent input is a send

lib/mutant/mutator/node/block.rb

@@ -22,6 +22,7 @@ module Mutant
           if body
             emit_body_mutations
           end
+          emit_body(nil)
           emit_body(RAISE)
         end
 

lib/mutant/mutator/node/define.rb

@@ -15,6 +15,7 @@ module Mutant
         def dispatch
           emit_arguments_mutations
           emit_body(RAISE)
+          emit_body(nil)
           emit_body_mutations if body
         end