Common custom chain bugs

[acolytec3]

While researching #2436, I encountered what I think are a couple of minor logic bugs:

• When the miner starts in the client, if your common instance's consensus is set to pow, it errors trying to access the PoA consensus algorithm's subproperty of period since the consensusConfig object in the common._chainParams is undefined for PoW (this would presumably result in the same error for PoS). This resolve by just returning an empty object {} if no configuration parameters are defined.
• I think we had some misplaced parentheses in cli in lines 588 - 591. The error should occur in any scenario where we provide custom genesis parameters and also pass in either network or networkId since those tell the common constructor to pull in predefined network genesis parameters.

[codecov]

Codecov Report

Merging #2448 (e2b8d2c) into master (0d398cb) will decrease coverage by 0.52%.
The diff coverage is 80.00%.

Additional details and impacted files

Flag	Coverage Δ
block	89.51% <ø> (ø)
blockchain	90.03% <ø> (ø)
client	87.45% <ø> (ø)
common	98.08% <80.00%> (-0.09%)	⬇️
devp2p	91.78% <ø> (ø)
ethash	∅ <ø> (∅)
evm	79.73% <ø> (ø)
rlp	?
statemanager	89.61% <ø> (ø)
trie	90.66% <ø> (ø)
tx	?
util	84.59% <ø> (ø)
vm	85.67% <ø> (ø)

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