[electrum] remove indentation levels in get_payment_request

Summary: Return early instead of keeping the return values in local variables until the bottom of the function. There is no behavior change.

Test Plan:
python test_runner.py
Note that this function is covered by unit tests (I added an assert False line and checked that several tests fail)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16414