Fix null transform attribute warning in Firefox.

Strictly speaking, the transform attribute should never be set to null;
it should be set to "translate(0)" for the identity transform, or it
should be removed using removeAttribute("transform").

Setting this attribute to null causes a warning to appear in Firefox:

  "Unexpected value null parsing transform attribute."

This occurs when using d3.transform(null), e.g. to interpolate to or
from a non-existent transform attribute (null).

This fix checks whether the value is null, and if so, returns an
identity transform.