Handle empty language overrides

[f1sh1918]

Is your feature request related to a problem? Please describe.
While initialization locale overrides will be applied for the defined languages
Currently for koblenz we only have one local de and en can't be found and an unhandled error is thrown

Describe the solution you'd like

• avoid setting overrides with language files that are not available

lear and concise description of any alternative solutions or features you've considered.

Additional context

flutter: Environment: local
[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Unable to load asset: "assets/koblenz/l10n/override_en.json".
The asset does not exist or has empty data.