The Ruby C API function 'rb_funcall' is used in various places in generated
code for invoking a Ruby method without parameters. The C function uses a
variadic parameter list for the arguments passed to Ruby, therefore in these
cases the list of variadic parameters is empty.
As an optimization Ruby may implement the 'rb_funcall' function as a macro
which however will not accept an empty list of arguments for '...' as of
C99 and C++11.
In order to prevent compiler warnings, this commit replaces all such
occurrences with a call to 'rb_funcall2' (which in its current name
'rb_funcallv' is invoked by the 'rb_funcall' macro anyway, at least for
Ruby 2.6.6).
