refactor: use CHECK_NONFATAL to avoid single-use symbol

src/rpc/mining.cpp

@@ -778,9 +778,7 @@ static RPCHelpMan getblocktemplate()
         }
         ENTER_CRITICAL_SECTION(cs_main);
 
-        std::optional<uint256> maybe_tip{miner.getTipHash()};
-        CHECK_NONFATAL(maybe_tip);
-        tip = maybe_tip.value();
+        tip = CHECK_NONFATAL(miner.getTipHash()).value();
 
         if (!IsRPCRunning())
             throw JSONRPCError(RPC_CLIENT_NOT_CONNECTED, "Shutting down");