assert(!action_mroot.empty()) in valid_t block_state::new_valid() should not be needed

[linh2931]

When running eosio_system_part1_tests/stake_unstake under Savanna,

I got
unit_test: .../libraries/chain/block_state.cpp:283: eosio::chain::valid_t eosio::chain::block_state::new_valid(const eosio::chain::block_header_state&, const digest_type&, const digest_type&) const: Assertion !action_mroot.empty()' failed``

spring/libraries/chain/block_state.cpp

Line 283 in a781609

assert(!action_mroot.empty());

should not be needed, as it is possible a block is empty.

[arhag]

We should not assume that we always have some action receipt in the block. The onblock can fail as part of the protocol in which case an empty block would have no action receipts.

We should create a new test with a test contract that only causes onblock to fail to reproduce this problem without all the other stuff that the system contract tests bring in.