Consider changing merge_list default behavior regarding default NULL value

[cderv]

The issue is regarding defining new format using output_format() but using rmarkdown::knitr_options() which define some NULL value as default, even if you want to inherit the base format. This will prevent doing it.

See Related discussions for context

• dev metadata key not honoured yihui/xaringan#368
• An output format argument with no default causes rmarkdown:::default_output_format to fail with cryptic error message. #2407
• Argument keep_md not inherited by custom output format that extends html_document() #1558