[SPARK-17811] SparkR cannot parallelize data.frame with NA or NULL in Date columns

## What changes were proposed in this pull request?
NA date values are serialized as "NA" and NA time values are serialized as NaN from R. In the backend we did not have proper logic to deal with them. As a result we got an IllegalArgumentException for Date and wrong value for time. This PR adds support for deserializing NA as Date and Time.

## How was this patch tested?
* [x] TODO

Author: Hossein <hossein@databricks.com>

Closes #15421 from falaki/SPARK-17811.