Allow VC to start without any validators (#1779)

## Issue Addressed

NA

## Proposed Changes

- Don't exit early if the VC is without any validators.
- When there are no validators, always create the slashing database (even without `--init-slashing-protection`).

validator_client/src/lib.rs

@@ -29,7 +29,7 @@ use http_api::ApiSecret;
 use initialized_validators::InitializedValidators;
 use notifier::spawn_notifier;
 use slashing_protection::{SlashingDatabase, SLASHING_PROTECTION_FILENAME};
-use slog::{error, info, Logger};
+use slog::{error, info, warn, Logger};
 use slot_clock::SlotClock;
 use slot_clock::SystemTimeSlotClock;
 use std::marker::PhantomData;
@@ -107,23 +107,30 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         .await
         .map_err(|e| format!("Unable to initialize validators: {:?}", e))?;
 
+        let voting_pubkeys: Vec<_> = validators.iter_voting_pubkeys().collect();
+
         info!(
             log,
             "Initialized validators";
             "disabled" => validators.num_total().saturating_sub(validators.num_enabled()),
             "enabled" => validators.num_enabled(),
         );
 
-        if validators.num_enabled() == 0 {
-            return Err("Cannot run with 0 enabled validators. \
-                        Please create or import validators before starting the validator client, \
-                        or check your datadir configuration"
-                .to_string());
+        if voting_pubkeys.is_empty() {
+            warn!(
+                log,
+                "No enabled validators";
+                "hint" => "create validators via the API, or the `lighthouse account` CLI command"
+            );
         }
 
         // Initialize slashing protection.
+        //
+        // Create the slashing database if there are no validators, even if
+        // `init_slashing_protection` is not supplied. There is no risk in creating a slashing
+        // database without any validators in it.
         let slashing_db_path = config.validator_dir.join(SLASHING_PROTECTION_FILENAME);
-        let slashing_protection = if config.init_slashing_protection {
+        let slashing_protection = if config.init_slashing_protection || voting_pubkeys.is_empty() {
             SlashingDatabase::open_or_create(&slashing_db_path).map_err(|e| {
                 format!(
                     "Failed to open or create slashing protection database: {:?}",
@@ -143,11 +150,11 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         // Check validator registration with slashing protection, or auto-register all validators.
         if config.init_slashing_protection {
             slashing_protection
-                .register_validators(validators.iter_voting_pubkeys())
+                .register_validators(voting_pubkeys.iter().copied())
                 .map_err(|e| format!("Error while registering slashing protection: {:?}", e))?;
         } else {
             slashing_protection
-                .check_validator_registrations(validators.iter_voting_pubkeys())
+                .check_validator_registrations(voting_pubkeys.iter().copied())
                 .map_err(|e| {
                     format!(
                         "One or more validators not found in slashing protection database.\n\