Server error thrown if no data at all returned from query to `ta_path`

[Nate-Wessel]

Find this error here for example:
http://localhost:8072/aggregate-travel-times/863505565/1260558203/9/10/2026-01-03/2026-01-25/true/67

There is (erroneously, but still) no data for this corridor in ta_path and it seems that polars instead of returning an empty dataframe with the correct datatypes inferred from the DB columns, returns an empty dataframe with NULL datatypes?

The error then happens here when it tries to join a string to a null?:

bdit_tt_request_app/backend/app/travel_times/get_travel_time.py

Lines 102 to 106 in 60bb4c0

	# join link lengths and
	# calculate link travel times from speed and length (in seconds)
	link_times_df = link_speeds_df.join(
	links_df, on='link_dir'
	).select( [

datatypes of join keys don't match - `link_dir`: null on left does not match `link_dir`: str on right