python.pkgs.wrapPython: get rid of warning

When `makeWrapperArgs` variable is not set, `declare -p makeWrapperArgs`
will return with 1 and print an error message to stderr.

I did not handle the non-existence case in b0633406
because I thought `mk-python-derivation` will always define `makeWrapperArgs`
but `wrapProgram` can be called independently. And even with `mk-python-derivation`,
`makeWrappers` will not be set unless explicitly declared in the derivation
because of https://github.com/NixOS/nix/issues/1461.

I was lead to believe that because the builds were succeeding and I confirmed
that the mechanism fails when the variable is not defined and `-o nounset` is enabled.
It appears that `wrapPython` setup hook is not running under `-o nounset`, though,
invaldating the assumption.

Now we are checking that the variable exists before checking its type, which
will get rid of the warning and also prevent future error when `-o nounset`
is enabled in the setup hook.

For more information, see the discussion at
https://github.com/NixOS/nixpkgs/commit/a6bb2ede232940a96150da7207a3ecd15eb6328