refactor: make code more readable and early exit

[polespinasa]

Overall the code looks good and I think it does what is intended to do.

It can be more clean and readable as this PR suggest. Long lines of code with a lot of class attributes calls and pointers are close to unreadable.
Also the if else structure is cleaner and avoid repeating calls such as IsMine() or PSBTInputSigned(). Normally we want to avoid duplicating code.

At the same time there is a small improvement. The only thing this loop does is check if the inputs are signed. At the moment we find one intput that is not signed we can early exit. It is impossible to set complete=true once it is changed to false.