JAVA-428: be consistent for setter

Author: agirbal

src/main/com/mongodb/DB.java

@@ -421,7 +421,6 @@ public com.mongodb.WriteConcern getWriteConcern(){
      * @param preference Read Preference to use
      */
     public void setReadPreference( ReadPreference preference ){
-        if ( preference == null ) throw new IllegalArgumentException();
         _readPref = preference;
     }