std.Build.Step.ConfigHeader: handle empty keys more permissively

[Jan200101]

${} is valid in cmake config headers and needs to be substituted instead of throwing an error

cmake_minimum_required(VERSION 3.15)

project(test)

set("" "Empty Key")

message("Output: ${}")

$ cmake .
Ouput: Empty Key
-- Configuring done (0.0s)
-- Generating done (0.0s)
-- Build files have been written to: /tmp/tmp.xjaLJ6qVKc

there are practically no real world projects that depend on this but it still needs to be supported.
There are still projects (e.g. darktable) with cmake config headers that include ${} in a comment that is not intended to be subsituted with anything else, I think that requiring the user to explicitly provide an empty key introduces unnecessary friction for a problem that isn't real, so this PR uses .undef as a fallback instead of erroring.

TL;DR

• make empty key not an error
• if empty key has no value default to undefined