title: rhandsontable writing NULL values to SQL database using dbWriteTable
body: <p>I'm creating the following table using library(rhandsontable)</p>
  
  <pre><code>rhandsontable(df, className = "htCenter", stretchH = "all", columnSorting = TRUE) %&gt;%
           hot_col("Date", type = "date", dateFormat = "YYYY-MM-DD") %&gt;%
           hot_cols(format = '1')
  </code></pre>
  
  <p>The table is converted to a data frame <code>dfHot &lt;- hot_to_r(input$hot)</code> and then copied to a SSMS SQL database with</p>
  
  <pre><code>dbWriteTable(con, "TEST", dfHot, append = TRUE, overwrite = FALSE, row.names = FALSE)
  </code></pre>
  
  <p>The problem is the table can contain <code>NULL</code> values which appear as <code>NA</code> in R, but I need them to come across as <code>NULL</code> into SQL.</p>
  
  <p>I'm getting this error when <code>NA</code> values are present in the table:</p>
  
  <blockquote>
    <p>Warning: Error in .local: execute JDBC update query failed in
    dbSendUpdate (The incoming tabular data stream (TDS) remote procedure
    call (RPC) protocol stream is incorrect. Parameter 6 (""): The
    supplied value is not a valid instance of data type float. Check the
    source data for invalid values. An example of an invalid value is data
    of numeric type with scale greater than precision.)</p>
  </blockquote>
  
  <p>If no <code>NA</code> values are present, the table imports without any issues.</p>