fix: define execution_info outside conditionals

[redeboer]

Causes a small bug in case no solutions are found. E.g.:

import qrules

reaction = qrules.generate_transitions(
    initial_state=("psi(4160)", [-1, +1]),
    final_state=["D-", "D0", "pi+"],
    allowed_intermediate_particles=[],  # <-- wrong, but should not crash
)

See also #33