Mark begin; end as empty expression

spec/ameba/rule/empty_expression_spec.cr

@@ -24,6 +24,12 @@ module Ameba
         a = nil
         a = ""
         a = 0
+
+        nil
+        :any.nil?
+
+        begin "" end
+        [nil] << nil
       )
       subject.catch(s).should be_valid
     end
@@ -69,6 +75,25 @@ module Ameba
         ()
       end
     )
+    it_detects_empty_expression %(
+      def method
+        begin
+        end
+      end
+    )
+    it_detects_empty_expression %(
+      begin; end
+    )
+    it_detects_empty_expression %(
+      begin
+        nil
+      end
+    )
+    it_detects_empty_expression %(
+      begin
+        ()
+      end
+    )
 
     it "reports rule, location and message" do
       s = Source.new %(

src/ameba/ast/traverse.cr

@@ -13,6 +13,7 @@ module Ameba::AST
     Def,
     EnumDef,
     ExceptionHandler,
+    Expressions,
     If,
     InstanceVar,
     LibDef,

src/ameba/rule/empty_expression.cr

@@ -42,5 +42,11 @@ module Ameba::Rule
 
       source.error self, node.location, "Avoid empty expression '#{exp}'"
     end
+
+    def test(source, node : Crystal::Expressions)
+      if node.expressions.size == 1 && node.expressions.first.nop?
+        source.error self, node.location, "Avoid empty expressions"
+      end
+    end
   end
 end