add empty value for keys without value

[emrekaraca]

Currently, when you add a new language key to a Lokalize-project, but leave the value empty, the key is only saved in keys but not in values of the project-entry.

When values are not set, they are not delivered by the Rest-API-endpoint, which can lead to inconsistencies.

The changes in this PR set any unset value to an empty string, which ensures that every entry in the Lokalize-project is also delivered from the Rest-API-endpoint.