Fix MWImageNode dimensions and implement toDomElements

Using element.height was returning 0 if the attribute was empty
when in fact what we mean to store is null (i.e. auto height).

This takes care of the writing of attributes in CE as jQuery
ignores an attribute-set command if the value is null.

Also in this commit I've implemented a basic toDomElements
that outputs the original HTML (code copied from AlienNode).
This stops the code from throwing an exception but will
eventually need to be rewritten to rebuild the HTML from
the attributes stored in the DM.

Bug: 56336
Change-Id: I297a1d0a07e9ebf9d0110fb1cdf266f8415f25b7