In ExtData2G, do not check for file file variable if collection is /dev/null

[bena-nasa]

I checked out main of the GEOSgcm fixture as I saw some of the repos were moved to newer version and did my usual does ExtData2G still work test. I found it was failing as some new variables were added to the TR grid comps ExtData files that are set to /dev/null for now. But those rules did not have a "variable" keyword as that's not needed in this case but the lack of this was causing a crash.
I went back and looked at my test case 19 that tests /dev/null and indeed it looks like I just took an existing file and changed the collection to /dev/null and left the variable there so that's why my test case 19 was working. But when I looked at the logic in ExtData2G, it was a bit screwy in that if the collection was /dev/null I still insisted the variable keyword be there. This is not necessary in that case and should be fixed.