[Snapps] Fix: Don't throw an exception on snapp commands with zero transfer

[mrmr1993]

Snapp commands may have 0 delta for both parties if there is a separate fee payer. This stops the code from throwing an exception.

The logic is still wrong: if a fee payment is given and different from either of the snapp accounts, the calculated total should be 0 regardless of the difference. Still, this is enough for now to make the snapp demo not fall at this hurdle.

Checklist:

• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them:

[mrmr1993]

Updated, turns out we need the more correct behaviour, so implemented that here.