R Markdown how to make parameter optional

Is there a way to make a parameter optional in R Markdown when using knitr? Below is a stand-alone example, my simple attempt to omit a parameter, and the error.

Example RMD file:

title: "`r params$name`"
date: \`r format(Sys.Date(), "%B %d, %Y")`\
    toc: true
    A: NA
    B: NA
    C: NA
# Header-1

`r params$A` 
`r params$B`
`r params$C`

Example run script:

render(input = "mytemplate.Rmd", 
       output_file = "fileout.html", 
       params = list(
         name = 'Bob', 
         A = 'turkey', 
         B = 'otter'

Resulting error when omitting "C":

Error in knitr::include_graphics(params$C) : 
  Cannot find the file(s): NA

Read more here: https://stackoverflow.com/questions/67479791/r-markdown-how-to-make-parameter-optional

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