The Haskell Hydra report generator
(`maintainers/scripts/haskell/hydra-report.hs`) uses this
`maintainer-handles.nix` script for generating a mapping of email
addresses to GitHub handles.
This `maintainer-handles.nix` script is necessary because the Haskell
Hydra report generator gets Hydra job status info as input, but needs to
ping users on GitHub. Hydra job status info only contains user emails (not
GitHub handles). So the `maintainer-handles.nix` script is necessary
for looking up GitHub handles from email addresses.
This commit fixes the `maintainers-handles.nix` code to ignore
maintainers that don't have email addresses. The code was originally
assuming that all maintainers have email addresses, but there was
recently a maintainer added without an email address.
