FindJNI: Improve output on macOS if Xcode is not installed

Improve the changes from:

* commit efa0e2ddaf (FindJNI: Update for Xcode greater than 12.1 on macOS,
                     2023-08-06, v3.27.2~5^2)
* commit 597139b853 (FindJNI: Restore support for macOS without Xcode,
                     2023-08-11, v3.27.3~8^2)

to avoid leaking `xcodebuild`'s stderr.

Issue: #26229

Modules/FindJNI.cmake

@@ -391,8 +391,10 @@ if(APPLE)
     execute_process(
       COMMAND xcodebuild -version
       OUTPUT_VARIABLE _FindJNI_XCODEBUILD_VERSION
+      ERROR_VARIABLE _FindJNI_XCODEBUILD_VERSION
+      RESULT_VARIABLE _FindJNI_XCODEBUILD_RESULT
       )
-    if(_FindJNI_XCODEBUILD_VERSION MATCHES "Xcode ([0-9]+(\\.[0-9]+)*)")
+    if(_FindJNI_XCODEBUILD_RESULT EQUAL 0 AND _FindJNI_XCODEBUILD_VERSION MATCHES "Xcode ([0-9]+(\\.[0-9]+)*)")
       set(_FindJNI_XCODE_VERSION "${CMAKE_MATCH_1}")
     else()
       set(_FindJNI_XCODE_VERSION "")