asio: object_server: fix 'callFunction' for void result
asio: object_server: fix 'callFunction' for void result

`auto r = ...` doesn't work when the expression returns a void.
Improve the 'callFunction' functions to handle the void result case
by putting the `m.append` into a separate constexpr-if.

Fixes eb6a9d0.

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: I31dfc23d123000c62efe6243b374ba65fc0d5abd

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus