## What changes were proposed in this pull request?

Several SparkR API calling into JVM methods that have void return values are getting printed out, especially when running in a REPL or IDE.
example:
```
> setLogLevel("WARN")
NULL
```
We should fix this to make the result more clear.

Also found a small change to return value of dropTempView in 2.1 - adding doc and test for it.

## How was this patch tested?

manually - I didn't find a expect_*() method in testthat for this

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16237 from felixcheung/rinvis.