Fix `\figure` output from markdown when the alternate text is empty

[gaborcsardi]

Reading 'man/markdown_pass1.Rd'
Error in if (substr(opt, 1, 9) == "options: ") { : 
  argument is of length zero
Calls: <Anonymous> ... flatten_para -> <Anonymous> -> .f -> as_html.tag_figure
In addition: Warning message:
In path[to_set] <- paste0(path_ext_remove(path[to_set]), ".", rep(ext,  :
  number of items to replace is not a multiple of replacement length
Execution halted
##[error]Process completed with exit code 1.

https://github.com/r-lib/roxygen2/runs/489923492