FindZLIB: fix CMAKE_FIND_LIBRARY_PREFIXES being unset when it was empty
CMAKE_FIND_LIBRARY_PREFIXES and CMAKE_FIND_LIBRARY_SUFFIXES have different behavior when undefined and when defined but empty: Empty means to use an empty prefix/suffix while undefined means to use a hardcoded default for the platform we are running on.
Unfortunately, set(a ${b}) will undefine a when b is empty, meaning that when targeting a platform where either of these variables is empty (e.g. Windows where CMAKE_FIND_LIBRARY_PREFIXES is empty) the unpatched FindZLIB code ends up unsetting that variable, causing all subsequent find_library calls to use the hardcoded default for the runtime platform (e.g. "lib" for CMAKE_FIND_LIBRARY_PREFIXES on Linux).
On the other hand, set(a "${b}") will always define a to be empty but defined so we have to do this dance to fully preserve the state of these variables.
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Other find modules that mess with CMAKE_FIND_LIBRARY_{PREFIXES,SUFFIXES} probably need a similar fix.
Note that while this is especially noticeable while cross-compiling, the existing FindZLIB code changes the effective value of CMAKE_FIND_LIBRARY_PREFIXES even on native Windows builds from "" set in Modules/Platform/Windows.cmake to the hardcoded ";lib" in Source/cmFindLibraryCommand.cxx.
Backport: release