[MINOR] [SPARKR] Fix some validation problems in SparkR

Getting rid of some validation problems in SparkR
https://github.com/apache/spark/pull/7883

cc shivaram

```
inst/tests/test_Serde.R:26:1: style: Trailing whitespace is superfluous.

^~
inst/tests/test_Serde.R:34:1: style: Trailing whitespace is superfluous.

^~
inst/tests/test_Serde.R:37:38: style: Trailing whitespace is superfluous.
  expect_equal(class(x), "character")
                                     ^~
inst/tests/test_Serde.R:50:1: style: Trailing whitespace is superfluous.

^~
inst/tests/test_Serde.R:55:1: style: Trailing whitespace is superfluous.

^~
inst/tests/test_Serde.R:60:1: style: Trailing whitespace is superfluous.

^~
inst/tests/test_sparkSQL.R:611:1: style: Trailing whitespace is superfluous.

^~
R/DataFrame.R:664:1: style: Trailing whitespace is superfluous.

^~~~~~~~~~~~~~
R/DataFrame.R:670:55: style: Trailing whitespace is superfluous.
                df <- data.frame(row.names = 1 : nrow)
                                                      ^~~~~~~~~~~~~~~~
R/DataFrame.R:672:1: style: Trailing whitespace is superfluous.

^~~~~~~~~~~~~~
R/DataFrame.R:686:49: style: Trailing whitespace is superfluous.
                    df[[names[colIndex]]] <- vec
                                                ^~~~~~~~~~~~~~~~~~
```

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #8474 from yu-iskw/minor-fix-sparkr.