Make source_account optional and use .source_account() to handle None case with config lookup

[willemneal]

          The only place where this `Address` is pulled from the config is for the source account, so we could just handle the option is the `source_account()`, method. If `self.source_account` is None, then we use the config, otherwise it will be the `Address` type and it resolves as normal. I do agree that using the empty string as a special case is not very idiomatic rust and Option is the preferred way to do it.  Plus my recent work adding a `ContractAddress` type make me realize that we need a unified approach between it and `Address` because in contract invoke an `ScAddress` could be either! So more reason not to handle the config at all in the `Address` type.

Originally posted by @willemneal in #1711 (comment)

[github-actions]

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