title: Allow null in unique column
body: <p>I've created the following table: </p>
  
  <pre><code>CREATE TABLE MMCompany (
     CompanyUniqueID BIGSERIAL PRIMARY KEY NOT NULL, 
     Name VARCHAR (150) NOT NULL,
     PhoneNumber VARCHAR(20) NOT NULL UNIQUE, 
     Email VARCHAR(75) UNIQUE,
     CompanyLogo BYTEA
   );
  </code></pre>
  
  <p>The email column is unique and it causes a "bug" in my scenario since there could only be one record with null. I'm trying to achieve records of companies without the same email but at the same time allow a companies to have no email.</p>
  
  <p>How can I achieve that?</p>