Fix unsafe check of the compiler in the CMakeToolchain generator (#9801)

The compiler variable here must not be a NoneType object to use `in`.
This will result in an error when attempting to package a header-only library.
This commit fixes this check.
It first verifies the compiler is not None before checking it.

conan/tools/cmake/toolchain.py

@@ -617,7 +617,7 @@ def context(self):
         compiler = self._conanfile.settings.get_safe("compiler")
         # TODO: Check if really necessary now that conanvcvars is used
         if (generator is not None and "Ninja" in generator
-                and ("Visual" in compiler or compiler == "msvc")):
+                and (compiler is not None and "Visual" in compiler or compiler == "msvc")):
             compiler = "cl"
         else:
             compiler = None  # compiler defined by default