Add a constant for "empty speak tag"

[sgellock]

We have agreed on a xml tag structure to indicate an empty speak tag. The format of that tag is:

Let's expose this string as a constant in the speech namespace.

For internal user, you can get additional data here:
microsoft/botframework-obi#85

Relates to:
https://github.com/microsoft/botbuilder-python/issues/1269

[enhancement]