Update webroot files to fix an issue in ServerShell.

The previous fixes done in 2918ae9c7ab86208be25d5652d94c14685c59faa
caused other issues preventing actions from being dispatched properly.

Fixes #3934
Add failing test for #3930
Throw exceptions when '' is used as translation domain.

'' is never a good translation domain and often indicates developer
error. Treat it as a mistake and throw an exception.

Fixes #3939
Fix FileEngine not clearing keys when groups are used.

Fixes #3930