Skip building R vignettes if Spark is not built

## What changes were proposed in this pull request?

When we build the docs separately we don't have the JAR files from the Spark build in
the same tree. As the SparkR vignettes need to launch a SparkContext to be built, we skip building them if JAR files don't exist

## How was this patch tested?

To test this we can run the following:
```
build/mvn -DskipTests -Psparkr clean
./R/create-docs.sh
```
You should see a line `Skipping R vignettes as Spark JARs not found` at the end

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #15200 from shivaram/sparkr-vignette-skip.