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

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• 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.

Makefile

@@ -80,6 +80,7 @@ conan: conan-install-deps conan-create
 .PHONY: conan
 
 doc:
+	@doxygen --version || ( echo "Doxygen not found.  Please install Doxygen first." ; exit 1 )
 	mkdir -p build && cd build && conan install .. --build=missing -s $(CONAN_SETTINGS) \
 	  && cmake .. && cmake --build . --target doc-sphinx \
 	  && echo "The documentation starts at build/doc/sphinx_out/index.html ."