EMSUSD-2867 - Ufe: modernize (c++17) Ufe::Value using std::variant

[seando-adsk]

EMSUSD-2867 - Ufe: modernize (c++17) Ufe::Value using std::variant

• Return empty Ufe::Value() when string is empty ("") as a Ufe::Value("") is not empty().

[seando-adsk]

This change is required to "fix" a problem I discovered in the Ufe::Value when porting it over to use std::variant. Previously Ufe::Value("").empty() would return True. That goes against what was written in the value.h docs. I fixed that with the std::variant changes and that exposed a couple of bugs in MayaUsd code.

I grep'd the entire repo for use of Ufe::Value and checked all the string cases.