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.

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doc/Install_develop.rst

@@ -28,6 +28,7 @@ package manager. The compiler must support the C++17 standard.
 1. clone/update the subrepo ``git submodule update --init proto``
 2. ``mkdir build && cd build``
 3. ``conan install .. -s "compiler.libcxx=libstdc++11"``
+   - When there are missing dependencies, it may help to run this with ``--build=missing``.
 4. ``cmake -B . ..``
 5. ``cmake --build .``