Allow empty messages and terms

[stasm]

Right now the following are syntax errors:

# Terms must have a value.
-term =
# Messages must have a value or at least one attribute.
message =

I wonder if it would make sense to make them valid syntax, even if they aren't valid translations. I'd expect tools like compare-locales to report errors about them and Pontoon to treat them as untranslated. But it might be useful to write down a list of identifiers to fill in later on and not have your editor throw errors on each line at you.

(Low priority food for thought.)

[zbraniecki]

I'm in favor of this change.

[Pike]

I think keeping these as parse errors is good.

They're in the way of runtime fallback, they're showing up as translated strings in pontoon, etc.

[flodolo]

I think keeping these as parse errors is good.

+1. Without concatenations, I don't expect us to have a lot of use cases for empty strings, most of the time those are just mistakes.

For the record, this helped me fix at least a dozen broken strings across locales in the last update.