Don't attempt to register validators that are pre-activation (#3441)

## Issue Addressed

https://github.com/sigp/lighthouse/issues/3440

## Proposed Changes

Don't consider pre-activation validators for validator registration. 



Co-authored-by: sean <seananderson33@gmail.com>
Co-authored-by: Michael Sproul <micsproul@gmail.com>

validator_client/src/preparation_service.rs

@@ -285,6 +285,9 @@ impl<T: SlotClock + 'static, E: EthSpec> PreparationService<T, E> {
 
     fn collect_validator_registration_keys(&self) -> Vec<ValidatorRegistrationKey> {
         self.collect_proposal_data(|pubkey, proposal_data| {
+            // Ignore fee recipients for keys without indices, they are inactive.
+            proposal_data.validator_index?;
+
             // We don't log for missing fee recipients here because this will be logged more
             // frequently in `collect_preparation_data`.
             proposal_data.fee_recipient.and_then(|fee_recipient| {