Allow default summary list value text to be configured

[peteryates]

Being able to set the default fallback text for summary list values is convenient and saves people from peppering extra logic throughout the views.

The new configuration value, default_summary_list_value_text, defaults to "" so the behaviour of the library won't change unexpectedly.

Fixes #411

Final tasks

• add a paragraph (or maybe new section) to the guide explaining the behaviour and suggesting better alternatives than fallback text following @frankieroberto's suggestion

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[frankieroberto]

Makes sense, but perhaps we should be nudging people towards this pattern instead: https://design-system.service.gov.uk/components/summary-list/#showing-missing-information - rather than having default text like "Not set"? 🤔

[peteryates]

Yeah, I agree. I think this option and the behaviour can easily be explained in a couple of paragraphs in the guide. Still, adding a link requires effort so being able to configure a fallback (even one that defaults to "") is likely to have utility I think.