[FEATURE]: Make `templateFormatString` fallback value configurable

[camdecoster]

Description

In templateFormatString, if a value is undefined, the format specifier is returned and a warning is logged to the console. This fallback value should be configurable so that a user may return "N/A" or some other message.

Why should this feature be added?

The current approach looks like a bug (being addressed in #7564) and doesn't provide any useful information to the viewer. Making it configurable would allow devs to better communicate to users what is happening with the missing data.

Mocks/Designs

Notes

• This issue was first discussed in plotly/plotly-studio#1504