cmake: Handle compiler.find_library returning None

I'm not 100% sure that warning is the appropriate action, but it's
better than failing with a backtrace.

Fixes: #12555

mesonbuild/cmake/tracetargets.py

@@ -55,7 +55,11 @@ def resolve_cmake_trace_targets(target_name: str,
                 # CMake brute-forces a combination of prefix/suffix combinations to find the
                 # right library. Assume any bare argument passed which is not also a CMake
                 # target must be a system library we should try to link against.
-                res.libraries += clib_compiler.find_library(curr, env, [])
+                flib = clib_compiler.find_library(curr, env, [])
+                if flib is not None:
+                    res.libraries += flib
+                else:
+                    not_found_warning(curr)
             else:
                 not_found_warning(curr)
             continue