fix(server): treat empty-string env vars as unset in config validation

[dnplkndll]

PRACTICE PR on fork — for quality review before filing upstream. Do not merge.

Addresses failure mode (1) of twentyhq#21035.

Compose passes VAR: ${VAR} for many optional variables, so any var unset on
the host reaches the server as '' — and boot validation rejects it instead of
applying the documented default:

$ docker run --rm -e STORAGE_TYPE='' ... twentycrm/twenty:latest
ERROR STORAGE_TYPE must be one of the following values:
ERROR [ExceptionHandler] ConfigVariableException [Error]: Config variables validation failed

validate() now strips empty-string entries before plainToClass (matching
compose's own "empty means unset" semantics) and logs which vars were treated
as unset. Explicit invalid non-empty values still fail as before.

Tests: valid value honored, compose-shaped batch of empties → defaults,
invalid non-empty still throws. Full twenty-config module: 198/198.