fix(lint): Lint/UnusedBlockArgument is triggered by abstract def

closes #352

spec/ameba/rule/lint/unused_block_argument_spec.cr

@@ -89,6 +89,13 @@ module Ameba::Rule::Lint
         CRYSTAL
     end
 
+    it "doesn't report if used in abstract def" do
+      expect_no_issues subject, <<-CRYSTAL
+        abstract def debug(id : String, &on_message: Callback)
+        abstract def info(&on_message: Callback)
+        CRYSTAL
+    end
+
     context "super" do
       it "reports if variable is not referenced implicitly by super" do
         source = expect_issue subject, <<-CRYSTAL

src/ameba/rule/lint/unused_block_argument.cr

@@ -47,6 +47,8 @@ module Ameba::Rule::Lint
     end
 
     def test(source, node : Crystal::Def, scope : AST::Scope)
+      return if node.abstract?
+
       return unless block_arg = node.block_arg
       return unless block_arg = scope.arguments.find(&.node.== block_arg)