improvement: use default value if the option is not configure in config.toml

[zzzckck]

System information

Geth version: v1.5.7
OS & Version: Windows/Linux/OSX
Commit hash : (if develop)

take DelayLeftOver for example:
it has default value: 50ms
https://github.com/bnb-chain/bsc/blob/v1.5.7/miner/minerconfig/config.go#L56

if a validator have the following setup in its config.toml

[Eth.Miner]
GasCeil = 140000000
GasFloor = 140000000
GasPrice = 1000000000
Recommit = 10000000000
VoteEnable = true

obviously, DelayLeftOver is not set.

Expected behaviour

• we expect it use the default 50ms

Actual behaviour

• but it is 0 actually

Steps to reproduce the behaviour

NA, mark: there are lots of such kind of cases, need to be improved.

Backtrace

NA

[zzzckck]

better to make config.toml as simple as possible.

[zzzckck]

more options better to enable the default value,
may refer this PR #3022

[Eth]
TrieTimeout = 600000000000
TriesInMemory = 128

[Eth.Miner]
Recommit = 3000000000

[Eth.Miner.Mev]
Enabled = true
BuilderFeeCeil= "0"

[Eth.TxPool]
Journal = "transactions.rlp"
Rejournal = 3600000000000
PriceLimit = 1
PriceBump = 10
AccountSlots = 16
GlobalSlots = 5120
AccountQueue = 64
GlobalQueue = 1024
Lifetime = 10800000000000

[Eth.GPO]
Blocks = 20
Percentile = 60
OracleThreshold = 1000

[Node.LogConfig]
TimeFormat = "01-02|15:04:05.000"

With default value, it could simplify the config.toml