disable 0-ary eta expansion

[adriaanm]

2.12.3 ticket: #196

[adriaanm]

This should set us on a path to fix the wart in scala/scala3#2570
(do eta-expansion even when the expected type is not known, except for 0-ary functions, where you must do it manually -- the default is to apply a zero-ary function to an empty argument list)

[SethTisue]

PR: scala/scala#6475