capture_output will return empty string, not nil

hsbt · hsbt · commit 3fa93f614431 · 2021-05-11T19:01:23.000+09:00
diff --git a/test/rubygems/test_gem_specification.rb b/test/rubygems/test_gem_specification.rb
@@ -3066,7 +3066,7 @@ def test_duplicate_runtime_dependency
     out, err = capture_output do
       @a1.add_runtime_dependency "b", "~> 3.0", "~> 3.0"
     end
-    assert_nil out
+    assert_empty out
     assert_equal(expected, err)
   end
 
