Update api_client.py

[ma1jmk]

Pull Request

Description

Fixes #64

Move the assignment outside the if statement to ensure change_to_dataframe is assigned even if query_params is empty.

Passing an empty list into the method self.format_to_dataframe(query_params) is fine.

How Has This Been Tested?

import elexonpy.api as elex
unit_data = elex.ReferenceApi().reference_bmunits_all_get()

# test endpoint with non-empty query_params
df = elex.DemandApi().demand_outturn_get(settlement_date_from='2025-09-01', settlement_date_to='2025-09-19', format='dataframe')

• [ - ] Yes

If your changes affect data processing, have you plotted any changes? i.e. have you done a quick sanity check?
Doesn't affect data processing

• Yes

Checklist:

• [- ] My code follows OCF's coding style guidelines
• [- ] I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• [- ] I have checked my code and corrected any misspellings

[peterdudfield]

Do all other tests pass? Sorry I havent set the CI to work on external PR yet

Update, got the CI running now