[R] Factor levels cannot contain NA

If you try to write a data frame with a factor with a missing value to parquet, you get the error: "Error: Invalid: Cannot insert dictionary values containing nulls". 

This seems likely due to how the metadata for factors is currently captured in parquet files. Reprex follows:

library(arrow)

bad_data <- data.frame(A = factor(1, 2, NA))

write_parquet(bad_data, tempfile())

**Reporter**: [Kieran Martin](https://issues.apache.org/jira/browse/ARROW-16768)

_{**Note**: *This issue was originally created as [ARROW-16768](https://issues.apache.org/jira/browse/ARROW-16768). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by running the provided R reprex with arrow's write_parquet() and inspect the factor metadata handling involved in writing Parquet data. Done means a data frame containing a factor with NA can be written without the dictionary-null error, with the expected missing value preserved.

Written by the indexing model from the issue text.