When is nil not false?

What about:
def initialize(source=nil)
  source ? xchg_source(source) : xchg_source
end

Or if you prefer longer notation
def initialize(source=nil)
  if source
    xchg_source(source)
  else
    xchg_source
end

Or like in your example:
def initialize(source=nil)
  return xchg_source unless source
  xchg_source(source)
end