Missing "return" statement in TopUpBalance._make_query

[flow-mr-t]

Describe the bug
A clear and concise description of what the bug is.
In the class TopUpBalance._make_query near the mention of "mode" the code snippet appears to need a 'return sql' that is not present. It not being present means the code tumbles through and overwrites the variable sql.

Product
Which part of FlowKit are you reporting an issue with?
FlowMachine

Version
Version(s) of the product affected.
Spotted in GitHub in the most recent file.

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

N/A - just look in the source code.

FlowKit/flowmachine/flowmachine/features/subscriber/topup_balance.py

Line 238 in 0f0da4c

Expected behavior
A clear and concise description of what you expected to happen.
I believe the code intends to return with the constructed sql.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.
Found as part of an investigation into work for World Bank (poverty)