A very good bug report. We will try to look into the matter within a week. Doesn't sound like a very hard fix.
But I took a look at the bounds-generation, I currently can't see the issue. I will have to debug it for real. Is that issue only in the -wpts file or in the "real" GPX file as well?
Was it in many different cases the attribute was =""? If it's only a few different variants, it would be great if you could edit your question and add examples of them. Then I don't have to try to reproduce, and maybe miss something.
Ok, a fix has been applied. After building a GPX with the cache GC3CF1X in it at Geocaching.com, I realize that they do not include waypoints with hidden coordinates at all. Project-GC is now doing the same, and therefore avoiding this issue (fix released to live servers a few minutes ago).
I also tested to build a GPX on 1200 random geocaches, there was no accourance of ="" in the GPX-files.