`cabal new-haddock` produces no documentation for single module executables

With the following .cabal file (and any Main.hs),
```
name: nodoc
version: 0
build-type: Simple
cabal-version: >= 1.2

executable nodoc
main-is: Main.hs
other-modules:
build-depends: base
```
no documentation is produced by `cabal new-haddock --haddock-executables`. As soon as one adds at least one module to the `other-modules` specification, documentation is generated, including the Main module.
```
$ cabal --version
cabal-install version 2.4.1.0
compiled using version 2.4.1.0 of the Cabal library
$ haddock --version
Haddock version 2.20.0, (c) Simon Marlow 2006
Ported to use the GHC API by David Waern 2006-2008
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.4.2
```