BitcoinClient should check for if the 'bitcoinWalletOrImportString' parameter is undefined

[mudiali]

https://github.com/decentralized-identity/sidetree/blob/a8ba6ec2a1fb809df55ce54e47339d8ac1e7df34/lib/bitcoin/BitcoinClient.ts#L47

If the admin forgets to add in either the import string or an actual IBitcoinWallet on this parameter then we should throw here. Currently, the this.bitcoinWallet member stays undefined and we throw when we try to use it.

Suggestion:

if (typeof bitcoinWalletOrImportString === 'undefined') { // throw }

// OR

// After all the initializations
if (this.bitcoinWallet === undefined) { // throw }