Was: tendermint/tendermint#7020

Part of: #1171

During tendermint/tendermint#6954 it was discovered that enterCommit does not check that the block that has received +2/3 precommits is non-nil. This check is performed outside of the call to enterCommit but should likely also be added to enterCommit to safeguard against the check not being included outside the method for future uses.

This issue was cloned from Tendermint Core repo as a follow up of #1175 (tracked by #1171)