Fix RETNIL macro for code

[PabstMirror]

#576 broke TRACE macros such as
TRACE_2("",_unit,local _unit); - due to isNil "local _unit" is true.
and
TRACE_1("",QUOTE(ADDON)); - quotation problems

Using the {} version of isNil seems to work just as well and fixes these problems.

[commy2]

BEEP. Reminder to myself to mute the machine when running it overnight.

This is right. This is how it was before we screwed it up by trying to use param. Thanks.

[654wak654]

😢 whoops