hasAssociation with empty file extension string {""} throws StringIndexOutOfBoundsException java
null file extension throws NullPointerException.
This is same as requestAssociation needs to change to either illegalArgumentException.

Suppose we have assocition like this
    <association>
      <mime-type="application-x/swingset2-file"/>
    </association>
If want to check hasAssocition("application-x/swingset2-file",null or {""}) ?
Is <extensions> are mandatory ?