New reports don't use 'title_series_line2' variable but older examples do; could confuse some users

[carlhiggs]

Describe the bug
If an older configuration file is used with newer code (enhancements branch) it can trip up if users don't modify example 'exception' citations, where reference is made to a phrase/variable 'title_series_line2'. This crashes the generate script, preventing a report being generated. The simple resolution is to put a check in the prepare_phrases() function, so if 'title_series_line2' is not defined, it can be defined as '-' which is pretty innocuous in a report title.

An example of an older excerpt of config yml that causes this issue is:

'citation_doi': '{local_collaborators_names}. 2022. {title_city} — {title_series_line1} {title_series_line2} ({city}, {country} — Healthy and Sustainable City Indicators Report: Comparisons with 25 cities internationally. {language} {translation}: {translation_names}). {city_doi}'

So in that context, with the fix an addition hyphen is included with a space on either side, seperating the report title and the city and country name.

This saves such users from having to modify their 'exception' citation, unless they want to.