Add capability to bypass beamer frame in section slides

[folofjc]

Describe your proposed improvement and the problem it solves.
Per #4317, content in a section header in markdown gets added to a new slide (frame) when converting to beamer. Since speaker notes are supported from markdown to beamer (#7856), I would like to add speaker notes to section slides (see ch 22 of the beamer manual). However, an empty frame is added around the \note{...} command in LaTeX. LaTeX can handle adding a speaker note to a section (see chapter 19 of the beamer manual). But I do not want the blank slide that pandoc adds when converting from markdown.

Describe alternatives you've considered.
See this discussion as well as the things that I tried and posted in this question.

[folofjc]

Thanks, @jgm!