Fix to return empty array when calling getEvents - Closes #7598

[shuse2]

What was the problem?

This PR resolves #7598

How was it solved?

Update to return empty array when the event does not exist for the height

How was it tested?

send transaction to dpos-mainchain example

./bin/run transaction:send 0a05746f6b656e12087472616e7366657218002080c2d72f2a20a3f96c50d0446220ef2f98240898515cbba8155730679ca35326d98dcfb680f032290a0810000000000000001080c2d72f1a143152da8f438d4591068553438c1a62edc2a161d4220227273a40cbefbdbeda4aa4d41e0a5f2887378ddec84ecf41f1227f029e442a82b3c29b56423c087354378368f719e55b1d8cc5e5ccc6aade8517d854c72799f2ca7cbd01

• call ./bin/run endpoint:invoke chain_getEvents --pretty '{"height": X }' for both height with event and height without event

[has5aan]

Non-existent block height results in [ ].
Transaction event for the concerned block height that included the transaction.