InstantSeal fix

ethcore/src/engines/instant_seal.rs

@@ -19,7 +19,7 @@ use util::Address;
 use builtin::Builtin;
 use engines::{Engine, Seal};
 use spec::CommonParams;
-use block::ExecutedBlock;
+use block::{ExecutedBlock, IsBlock};
 
 /// An engine which does not provide any consensus mechanism, just seals blocks internally.
 pub struct InstantSeal {
@@ -56,8 +56,8 @@ impl Engine for InstantSeal {
 
 	fn seals_internally(&self) -> Option<bool> { Some(true) }
 
-	fn generate_seal(&self, _block: &ExecutedBlock) -> Seal {
-		Seal::Regular(Vec::new())
+	fn generate_seal(&self, block: &ExecutedBlock) -> Seal {
+		if block.transactions().is_empty() { Seal::None } else { Seal::Regular(Vec::new()) }
 	}
 }