Due to how Python converts ints to bool the numeric argument ``0`` to
the :dedent: option of the code-block directive was handled like if no
argument was given to :dedent:.

Check properly for None in the options argument processing, so ``0`` is
treated as numeric argument to :dedent:.

Bug: sphinx-doc#9636

If the option :dedent: (without numeric argument) was given to
code-block then all newlines where striped from the resulting code.
Preserve newlines when stripping common whitespace at the beginning of
code blocks.

The presence or absence of the ``:dedent: 4`` option in the example

.. code-block:: ruby
   :dedent: 4

       some ruby code

makes no difference for the output due to how ReST handles whitespace.
If it is present, then ReST indent is 3 and the 4 spaces of the code are
stripped by the :dedent: option.  If the dedent option is missing, then
the ReST indent is 7 and all whitespace is stripped again.

Add a dummy option to fixate the ReST indent to 3, so adding and
removing of the option in the example has an effect on the output.

`:dedent: 0` option for the code-block directive is a secret feature.
So it should not be mentioned in our docs.