util: fix a misnamed local variable declaration.

* specl/util.lua (type_error): Declare `expected` correctly.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
util: matching a nil parameter with "any" type_check is ok!

* specl/util.lua (type_check): Lua treats deliberate `nil`
function arguments the same as omitted arguments, so don't raise
an error for a `nil` argument matched against an "any" type.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>