🥅 Handles zero-row tables more elegantly

[francisbarton]

This PR:

• adds functions that help return a zero-row tibble with the same columns and data classes as the tibble that is returned when the function is successful. This could happen, for example, if a filter to site codes includes non-existent/inapplicable site codes, or if a non-existent combination of parameters is supplied (eg activity type = "op" and measure = "admissions"). This helps reduce the surface for potentially deeply-buried or cryptic error messages. These guards are applied in appropriate places in the data preparation functions. The 0-row tibbles are returned with a "reason" attribute that can be read by error-handling functions that return a message.
• zero-row behaviour guards are also implemented in the various table and plot creation functions, so that instead of erroring they will produce a simple almost blank table or plot with the reason displayed.
• a suite of tests for these behaviours is also introduced

The concepts behind the execution of these guard functions and messaging outputs were generated by an LLM, but the code has since been examined and updated by me. The package at the point of this PR passes devtools::check().