Fix find opensubdiv condition check

[Danielmelody]

Description of Change(s)

Cmake would define OPENSUBDIV_${UPPERCOMPONENT}_LIBRARY to OPENSUBDIV_${UPPERCOMPONENT}_LIBRARY-NOTFOUND if find opensubdiv failed, this variable is definded, but evals to false, so we should check the variable itself, not whether it is defined.
Ref: https://cmake.org/cmake/help/latest/command/find_library.html

Fixes Issue(s)

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[tallytalwar]

Filed as internal issue #USD-7629