Be a bit more careful dealing with release list fetching failure

Author: expipiplus1

CMakeLists.txt

@@ -137,7 +137,7 @@ if(SLANG_SLANG_LLVM_FLAVOR MATCHES FETCH_BINARY)
     # If the user didn't specify a URL, find the best one now
     if(NOT SLANG_SLANG_LLVM_BINARY_URL)
         get_best_slang_binary_release_url(url)
-        if(NOT url)
+        if(NOT DEFINED url)
             message(FATAL_ERROR "Unable to find binary release for slang-llvm, please set a different SLANG_SLANG_LLVM_FLAVOR or set SLANG_SLANG_LLVM_BINARY_URL manually")
         endif()
     endif()

cmake/GitHubRelease.cmake

@@ -94,6 +94,7 @@ function(get_best_slang_binary_release_url out_var)
     set(repo "slang")
 
     check_release_and_get_latest(${owner} ${repo} ${SLANG_VERSION_NUMERIC} ${os} ${arch} release_version)
-
-    set(${out_var} "https://github.com/${owner}/${repo}/releases/download/v${release_version}/slang-${release_version}-${os}-${arch}.zip" PARENT_SCOPE)
+    if(DEFINED release_version)
+      set(${out_var} "https://github.com/${owner}/${repo}/releases/download/v${release_version}/slang-${release_version}-${os}-${arch}.zip" PARENT_SCOPE)
+    endif()
 endfunction()