title: Postgresql COPY empty string as NULL not work
body: <p>I have a CSV file with some integer column, now it 's saved as "" (empty string).</p>
  
  <p>I want to COPY them to a table as NULL value.</p>
  
  <p>With JAVA code, I have try these:</p>
  
  <pre><code>String sql = "COPY " + tableName + " FROM STDIN (FORMAT csv,DELIMITER ',',  HEADER true)";
  String sql = "COPY " + tableName + " FROM STDIN (FORMAT csv,DELIMITER ',', NULL ''  HEADER true)";
  </code></pre>
  
  <p>I get: PSQLException: ERROR: invalid input syntax for type numeric: ""</p>
  
  <pre><code>String sql = "COPY " + tableName + " FROM STDIN (FORMAT csv,DELIMITER ',', NULL '\"\"'  HEADER true)";
  </code></pre>
  
  <p>I get: PSQLException: ERROR: CSV quote character must not appear in the NULL specification</p>
  
  <p>Any one has done this before ?</p>