[Media TTT] Use a default name if blank string is given

Adds a default device name if the given routeInfo name is blank. Name
routeInfo builder does not accept empty or null string. Checking if the
name is blank should be fair enough in order to provide a reasonable string.

Bug: 261491161
Test: atest MediaTttSenderCoordinatorTest
Change-Id: Ibf38af23990d6ccf8f81eb6bcebc2ef0cf3f5d48