Fix weird tangle edge case

[jrnold]

This fixes a weird edge case for purl() / knit(tangle = TRUE). If there is no recognizable extension and no patterns are found in the input, then knit() will exit early with the original text. However, this behavior doesn't make sense when tangle = TRUE. This PR updates the knit() function to return "" in those cases.

library("knitr")

tangle_text = function(text) {
  # ensure that knit patterns is empty
  opat <- knit_patterns$get()
  knit_patterns$restore()
  on.exit(knit_patterns$restore(opat))
  knit(text = text, tangle = TRUE)
}

tangle_text("There is no code.")
#> [1] "There is no code."

devtools::session_info()
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#>  date     2019-01-19                  
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^{Created on 2019-01-19 by the reprex package (v0.2.1)}

[yihui]

The actual fix 326b77c looks good to me, but could you revert 3968f5f? I actually read this PR when you sent it in January, but hesitated because of the irrelevant cosmetic changes, which are unnecessary because these lines will be automatically wrapped when I build the package in RStudio:

• https://github.com/yihui/knitr/blob/master/knitr.Rproj#L19
• https://github.com/yihui/knitr/blob/master/man/engine_output.Rd

Lastly, could you add a news item to NEWS.md? Thanks a lot!