types: Coin.Validate should also check that .Amount is not nil before trying to deference it; not only check that .Denom is valid

[odeke-em]

Summary of Bug

Reported offline by @zanicar in a scenario in which they sent a JSON payload with an amount that can't JSON unmarshal
{"Denom": "stake", "Amount": 10000}, that value when JSON unmarshalled silently throws Amount away but saves .Denom and the code in .Validate() solely checks that .Denom is valid per

cosmos-sdk/types/coin.go

Lines 42 to 52 in 445dc8a

	func (coin Coin) Validate() error {
	if err := ValidateDenom(coin.Denom); err != nil {
	return err
	}
	if coin.Amount.IsNegative() {
	return fmt.Errorf("negative coin amount: %v", coin.Amount)
	}
	return nil
	}

Version

445dc8a

Remedy

We should also check that .Amount is not nil before trying to dereference it