[CMAKE] Fix the prl conversion script linking against lib_NOTFOUND

Summary:
If a component has no INTERFACE_LINK_LIBRARIES (empty property), the
script ends up linking against -l_link_libs_NOTFOUND due to the
required string conversion of the library path variable.
This diff resets the string representation to an empty string in this
case to avoid the linkage error.

Test Plan:

cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake -GNinja \
-DBUILD_BITCOIN_SEEDER=OFF ..
ninja

Check the build is successful.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2949