EXPORT_PROPERTIES: Add test for an undefined property

The added test verifies that a property listed in EXPORT_PROPERTIES
can be undefined. It confirms the crash recorded in issue #18260.

Tests/RunCMake/export/ExportPropertiesUndefined.cmake

@@ -0,0 +1,11 @@
+enable_language(CXX)
+add_library(foo empty.cpp)
+set_target_properties(foo PROPERTIES
+  EXPORT_PROPERTIES "NotDefinedProp"
+)
+export(TARGETS foo FILE "${CMAKE_CURRENT_BINARY_DIR}/foo.cmake")
+install(TARGETS foo EXPORT fooExport
+  RUNTIME DESTINATION bin
+  LIBRARY DESTINATION lib
+  ARCHIVE DESTINATION lib
+)

Tests/RunCMake/export/RunCMakeTest.cmake

@@ -8,3 +8,4 @@ run_cmake(NoExportSet)
 run_cmake(ForbiddenToExportInterfaceProperties)
 run_cmake(ForbiddenToExportImportedProperties)
 run_cmake(ForbiddenToExportPropertyWithGenExp)
+run_cmake(ExportPropertiesUndefined)