Avoid 0x00 block hashes in fcU (#9233)

- Avoid sending 0x00 block hashes for the safe and finalized block hashes post-Gloas.
- Add code to check this inside the mock EL, which will be reached in all Gloas beacon chain tests


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/execution_layer/src/test_utils/execution_block_generator.rs

@@ -69,6 +69,13 @@ impl<E: EthSpec> Block<E> {
         }
     }
 
+    pub fn timestamp(&self) -> u64 {
+        match self {
+            Block::PoW(block) => block.timestamp,
+            Block::PoS(payload) => payload.timestamp(),
+        }
+    }
+
     pub fn total_difficulty(&self) -> Option<Uint256> {
         match self {
             Block::PoW(block) => Some(block.total_difficulty),
@@ -558,6 +565,23 @@ impl<E: EthSpec> ExecutionBlockGenerator<E> {
             self.insert_block(Block::PoS(payload))?;
         }
 
+        // Post-Gloas, the justified and finalized block hashes must be non-zero, since the
+        // CL always has a known parent_block_hash to reference.
+        if let Some(head_block) = self.blocks.get(&head_block_hash)
+            && self
+                .get_fork_at_timestamp(head_block.timestamp())
+                .gloas_enabled()
+        {
+            assert!(
+                forkchoice_state.safe_block_hash != ExecutionBlockHash::zero(),
+                "post-Gloas safe_block_hash must not be zero"
+            );
+            assert!(
+                forkchoice_state.finalized_block_hash != ExecutionBlockHash::zero(),
+                "post-Gloas finalized_block_hash must not be zero"
+            );
+        }
+
         let unknown_head_block_hash = !self.blocks.contains_key(&head_block_hash);
         let unknown_safe_block_hash = forkchoice_state.safe_block_hash
             != ExecutionBlockHash::zero()