Prevent selser of zero-length DSR chunks.

We use zero-length TSRs to mark synthetic chunks of the DOM, for example
the <meta typeof="mw:Category"> generated by '#REDIRECT [[Category:Foo]]'.
We want to make sure that we don't try to selser these chunks if they
are edited.

Makes one more selser test pass, more-or-less randomly.

Change-Id: I07cf0b5432ed188f8d33eda8178d205f48f36b82
Merge "Prevent selser of zero-length DSR chunks."