Don't call SetMinSize() when creating the window if no initial size was given.

Don't call SetMinSize() when creating the window if no initial size was given.

Calling SetMinSize() is unnecessary in this case. It also results in GTK+
errors when creating wxFileDialog as it is not created yet when this is called
(but it does take care to pass wxDefaultSize to this function as its size
can't be set yet).

See r62814 and r62817.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62999 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775