[R] Empty character attributes not stored

Date-times in the POSIXct format have a 'tzone' attribute that by default is set to "", an empty character vector (not NULL) when created.

This however is not stored in the Arrow feather file. When the file is read back, the original and restored dataframes are not identical as per the below reprex.

I am thinking that this should not be the intention? My workaround at the moment is making a check when reading back to write the empty string if the tzone attribute does not exist.

Just to confirm, the attribute is stored correctly when it is not empty.

Thanks.
```java

``` r
dates <- as.POSIXct(c("2020-01-01", "2020-01-02", "2020-01-02"))
attributes(dates)
#> $class
#> [1] "POSIXct" "POSIXt"
#>
#> $tzone
#> [1] ""

values <- c(1:3)
original <- data.frame(dates, values)
original
#> dates values
#> 1 2020-01-01 1
#> 2 2020-01-02 2
#> 3 2020-01-02 3

tempfile <- tempfile()
arrow::write_feather(original, tempfile)

restored <- arrow::read_feather(tempfile)

identical(original, restored)
#> [1] FALSE

waldo::compare(original, restored)
#> `attr(old$dates, 'tzone')` is a character vector ('')
#> `attr(new$dates, 'tzone')` is absent

unlink(tempfile)
```
```
 

**Environment**: Ubuntu 20.04 R 4.1 release
**Reporter**: [Charlie Gao](https://issues.apache.org/jira/browse/ARROW-13588)

<sub>**Note**: *This issue was originally created as [ARROW-13588](https://issues.apache.org/jira/browse/ARROW-13588). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*</sub>

Start by running the R reprex in the issue to reproduce the Feather round-trip mismatch. Then trace how R attributes are written and restored during Feather I/O. Done means an empty POSIXct tzone attribute survives read-back and identical(original, restored) returns TRUE.

Written by the indexing model from the issue text.