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

@@ -189,11 +189,14 @@ For the tests there is a slightly different setup required (with option
     - Depending on the clang version it may be necessary to also add
       ``-DCMAKE_CXX_FLAGS="-Wno-unused-parameter"``
 4. ``cmake --build .``
+    - If this fails with ``Error running '': No such file or directory``, you may want to try
+      CMake's ``-D SKIP_LINTING=ON``. (See previous step.)
 
 Run
 ^^^
 
-In the build directory, run ``ctest``
+In the build directory, run ``ctest``.  For more verbose output of a single test:
+``ctest -R test_value.test_list -V``
 
 Framework
 ^^^^^^^^^