`compare_two_records` needs to check whether tf tables exist

[RobinL]

See https://github.com/moj-analytical-services/splink/discussions/769_

Hello! I've been trying to make use of the comparing two records feature, but I keep getting the following error: does anyone have any ideas what I'm doing wrong/ what to fix?

Originally posted by @mmagoffin-sd in https://github.com/moj-analytical-services/splink/discussions/769

[RossKen]

@ThomasHepworth has code from a similar issue in a PR (will be linked below)

[ThomasHepworth]

I believe this is partially resolved in here.

This should be relatively simple to resolve if we can reuse the code implemented above.

[RobinL]

It's harder than it looks because if the user just wants to compare two records, there's no guarantee they've provided Splink with a full input datasets. They might have just loaded in two records. If there is no full input dataset, we can't compute the tf tables. Instead, the user would have to provide them.

[James-Osmond]

Getting the same error trying to compare a record pair, without tf tables available

[RobinL]

Fixed in Splink4, see https://github.com/moj-analytical-services/splink/pull/2111