title: u-sql fails to handle null
body: <p>I am trying out azure datalake, and tried to test u-sql on my test file.</p>
  
  <p>this is my test script</p>
  
  <pre><code>@input =
      EXTRACT TagName     string,
              time        DateTime,
              valuee      float,
              Quality     int
      FROM "/MCVD/{*}/{*}/{*}/{*}/{*}/{*}/{*}.csv"
      USING Extractors.Csv(skipFirstNRows:1, nullEscape:"");
  
  OUTPUT @input
      TO "/output/Test-query.csv"
      USING Outputters.Csv();
  </code></pre>
  
  <p>When running in datalake analytics, i get an error with the column valuee at line 56.</p>
  
  <p>This is the content of line 56</p>
  
  <pre><code>"M32_Amp_FurnaceAluTemperatureTubeReference",2018-02-08T14:48:53.5780000,"",1
  </code></pre>
  
  <p>I understand that this is because the column holding the value is empty (its the first empty line in the file). I tried to use various NullEscape paterns, but it continues to fail. </p>
  
  <p>I tried to extract the column as a string, which didn't produce any errors.</p>
  
  <p>this is the error:</p>
  
  <pre><code>Vertex failed
  Vertex failure triggered quick job abort. Vertex failed: SV1_Extract[0][0] with error: Vertex user code error.
   VertexFailedFast: Vertex failed with a fail-fast error
   E_RUNTIME_USER_EXTRACT_ROW_ERROR: Error occurred while extracting row after processing 56 record(s) in the vertex' input split. Column index: 2, column name: 'valuee'.
   E_RUNTIME_USER_EXTRACT_COLUMN_CONVERSION_EMPTY_ERROR: Failure when attempting to convert empty column data.
  </code></pre>