[committed] wrong handling of empty attrs in DOMInputCapsule

At least 3 of the read methods in DOMInputCapsule can never use their specified "defVal" parameter, because it seems a previous author didn't know that org.w3c.dom.Element.getAttribute(String) returns empty String not null String if the sought-for attribute is not set.  In at least one place there is absolutely no attempt to use "defVal".  It's misleading to take an input parameter and ignore it.

I would just correct this, and it should be fixed, but the fix will change return values when reading jME XML files with missing or empty attribute values for a few high-level attr types (including the common String variety).

I will make this change, after unit and regression testing, if nobody objects within 24 hours.

UPDATE:  I'll update Spatial.read() at the same time to make Spatial.name behave as before, because I suspect we have a lot of code that implicitly depends on Spatial.name being non-null.

Committed with rev. 4540.

Also removed about 25 unused local variables and useless assignments while in there.  The results of indiscriminate copy & pasting.

Please do post here or PM me if anybody using jME XML files starts getting NPEs.