CMake: Fix automatic iOS bundle id extraction

If no Xcode preferences file was found,
_qt_internal_get_ios_bundle_identifier_prefix accidentally assigned
the error output of running PlistBuddy to the bundle id property.

That resulted in a bundle id called
'${team_id}.Doesn't Exist, Will Create: /Users/qt/Library/Preferences/com.apple.dt.Xcode.plist\n.app'

Check that the error variable is empty before assigning.

Amends 4d838dae5a

Pick-to: 6.1 6.2
Task-number: QTBUG-95838
Change-Id: I3a7241528590ae3e9986cfa1f3e91ac983ef54f6
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/corelib/Qt6CoreMacros.cmake

@@ -701,7 +701,7 @@ function(_qt_internal_get_ios_bundle_identifier_prefix out_var)
         message(DEBUG "Failed to extract the default bundle indentifier prefix.")
     endif()
 
-    if(prefix)
+    if(prefix AND NOT prefix_error)
         set_property(GLOBAL PROPERTY _qt_internal_ios_bundle_identifier_prefix "${prefix}")
         set("${out_var}" "${prefix}" PARENT_SCOPE)
     endif()