[CMake] Fix incorrect rpath for tests if LLVM_LOCAL_RPATH isn't set

d9ce31ae (D94322) removed the check because I thought it was dead
due to checking the existance of a variable (which always existed).

This causes LLDB tests to fail as they set NO_INSTALL_RPATH because
they're never meant to be installed, but we still would end up using
the install rpath.

Add the check back and make it explicitly check for an empty value
to make the purpose clearer and avoid implicit test for a false/true
value.

Differential Revision: https://reviews.llvm.org/D94326