Removing empty keys from GrlData

["Juan A." Suárez Romero <jasuarez igalia com>]

Hello.

For different reasons that don't matter right now, our GrlData API was
allowing to add keys without a value.

Thus, when getting the list of keys in a GrlData, people needed to check
if the key had a real value.

Nowadays, that doesn't make any sense, and in fact I think it is
confusing people.

So I'm going to remove that possibility from GrlData. That is, all keys
in GrlData will need to have a value.

I think this change, besides being an improvement about
misunderstandings, will not affect the clients nor the plugins.


	J.A.