Using Template Haskell, one can construct lambda expressions with no
arguments. The pretty-printer isn't aware of this fact, however. This
changes that.

Test Plan: make test TEST=T13856

Reviewers: bgamari, austin, goldfire

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13856

Differential Revision: https://phabricator.haskell.org/D3664