[MLIR][Python] Avoid printing an unset nanobind include directory (#211240)

`nanobind_INCLUDE_DIR` is only populated when MLIR discovers nanobind
through the Python package. When `nanobind_DIR` is supplied explicitly,
the variable remains unset and the configuration output contains an
empty path:
```
  Found nanobind v2.13.0:
```
The nanobind CMake package location is already reported by both
discovery paths.

We can only print the detected nanobind version in the final status
message. Or maybe if we want we can find the include dir for the
explicit path too and merge them to `nanobind_INCLUDE_DIR` but I think
this is better.

mlir/cmake/modules/MLIRDetectPythonEnv.cmake

@@ -82,6 +82,6 @@ macro(mlir_configure_python_dev_packages)
       set(nanobind_INCLUDE_DIR "${PACKAGE_DIR}")
     endif()
     find_package(nanobind 2.9 CONFIG REQUIRED)
-    message(STATUS "Found nanobind v${nanobind_VERSION}: ${nanobind_INCLUDE_DIR}")
+    message(STATUS "Found nanobind v${nanobind_VERSION}")
   endif()
 endmacro()