(PUP-2883) Allow empty body in epp templates

This allows an epp body to be empty by making grammar accept an
empty list of statements. This avoids getting a cryptic error message
about "syntax error at end of file" when using parameters, but having no
body that produces output, or when having a completely empty template.
Merge pull request #2868 from hlindberg/PUP-2883_empty-epp-template

(PUP-2883) Allow empty body in epp templates