Avoid NPE while checking header equality

While applying Parameters#isEqual(Object) method...

Parameters mergedClauses = OSGiHeader.parseHeader( oldValue );
Parameters clauses = OSGiHeader.parseHeader( value );
if ( !mergedClauses.isEqual( clauses) ) ...

...I ended up with NPE as follows

java.lang.NullPointerException
        at aQute.bnd.header.Attrs.isEqual(Attrs.java:261)
        at aQute.bnd.header.Parameters.isEqual(Parameters.java:200)