Fix default backend that I broke in fcfb47b3.

Change-Id: I3b7398f8fbfa739b1871156aad6031a05e801114
WTS: Accept '' extsrc in data-mw for extensions.

* Eliminates errors for "<math></math>" and the like.

  No change in serialized output itself since it serialized
  correctly previously as well.

Change-Id: Ieeea7fcf6e431dc3216278c3e01806e2cb1b7449
Merge "Fix default backend that I broke in fcfb47b3."
Merge "WTS: Accept '' extsrc in data-mw for extensions."