types.Coins hardening: NewCoin and NewDecCoin should fail if denom is empty

[alessio]

NewCoin and NewDecCoin functions should panic if the supplied denomination is empty in order to reduce surface of vulnerabilities that could potentially be introduced by lack of preemptive validation (e.g. by not calling IsValid())

On the other hand, ParseCoin functions should check for empty denoms and appropriately handle empty denoms zero-coin cases as per argument supplied by the caller. ParseCoin functions might take an additional parameter strict for instance to either return a ZeroCoin() or fail in zero-coin cases.
Alternatively, we may want to provide an alternative implementation for each case, e.g. ParsePositiveCoins, ParseNonNegativeCoins - either ways the client would not be required to call IsZero or IsPositive on the return value.

/cc @alexanderbez @cwgoes @sunnya97 @jackzampolin

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[cwgoes]

Sure; we should also harden the UnmarshalJSON functions if we want to defensively-code in this way.

[alexanderbez]

Adding validation is great to the new coin constructors! But ParseCoin/s already does validate denom via the regex: [[:alpha:]][[:alnum:]]{2,15}

[jackzampolin]

I like all these defensive coding ideas.

[cwgoes]

Adding validation is great to the new coin constructors! But ParseCoin/s already does validate denom via the regex: [[:alpha:]][[:alnum:]]{2,15}

Ah excellent, let's use the same validation in the NewXYZ constructors then, as long as it isn't too expensive.