Quick fix for eth_feeHistory when reward is nil (evmos#975)

* Quick fix for eth_feeHistory when reward is nil

`RPC method eth_feeHistory crashed: runtime error: invalid memory address or nil pointer dereference`
from https://github.com/tharsis/ethermint/blob/caa1c5a6c6b7ed8ba4aaf6e0b0848f6be5ba6668/rpc/ethereum/backend/feebackend.go#L29

* Update CHANGELOG.md

Author: loredanacirstea

CHANGELOG.md

@@ -37,6 +37,10 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ## Unreleased
 
+### Bug Fixes
+
+* (rpc) [#970] (https://github.com/tharsis/ethermint/pull/970) Fix unexpected nil reward values on `eth_feeHistory` response
+
 ### API Breaking
 
 * (ante) [\#866](https://github.com/tharsis/ethermint/pull/866) `NewAnteHandler` constructor now receives a `HandlerOptions` field.

rpc/ethereum/backend/feebackend.go

@@ -87,6 +87,9 @@ func (e *EVMBackend) processBlock(
 			}
 			tx := ethMsg.AsTransaction()
 			reward := tx.EffectiveGasTipValue(blockBaseFee)
+			if reward == nil {
+				reward = big.NewInt(0)
+			}
 			sorter[i] = txGasAndReward{gasUsed: txGasUsed, reward: reward}
 			break
 		}