ENH: Values can hold empty vector

Summary

Values should be able to hold empty vector. Before this MR, creating a value with an empty vector does not make this a vector.

See #76 (closed).

Focus

Nothing special.

Test Environment

Run the unit test.

Check List for the Author

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
  • How do I use the software? Assume "stupid" users.
  • How do I develop or debug the software? Assume novice developers.
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.

test/test_value.cpp

@@ -141,7 +141,7 @@ TEST(test_value, test_list) {
   auto empty_content = std::vector<int>();
   Value value_0(empty_content);
   EXPECT_TRUE(value_0.IsVector());
-  auto list_value_0 = value_0.GetAsVector();
+  auto const &list_value_0 = value_0.GetAsVector();
   EXPECT_EQ(list_value_0.size(), 0);
   EXPECT_TRUE(list_value_0.empty());
 }