GetPrerequisites: Restore behavior on missing binary of not clearing list

Prior to commit v3.4.0-rc1~264^2~1 (GetPrerequisites: Add error checks
for execute_process() calls, 2015-07-29), `get_prerequisites` would
simply warn on a missing binary and not update the result list at all.
That commit accidentally made the case an error.  This was fixed by
commit v3.8.0-rc1~110^2 (GetPrerequisites: Do not fail on files we
cannot find, 2017-01-10), but the fix also cleared the result list.
Clearing the list is incorrect because it is supposed to be able to
accumulate results over multiple calls.

Remove the list clearing behavior to restore the original behavior on a
missing binary.

Fixes: #17306

Modules/GetPrerequisites.cmake

@@ -659,7 +659,6 @@ function(get_prerequisites target prerequisites_var exclude_system recurse exepa
 
   if(NOT EXISTS "${target}")
     message("warning: target '${target}' does not exist...")
-    set(${prerequisites_var} "" PARENT_SCOPE)
     return()
   endif()
 

Tests/RunCMake/CMakeLists.txt

@@ -147,6 +147,7 @@ endif()
 add_RunCMake_test(GeneratorExpression)
 add_RunCMake_test(GeneratorPlatform)
 add_RunCMake_test(GeneratorToolset)
+add_RunCMake_test(GetPrerequisites)
 add_RunCMake_test(GNUInstallDirs -DSYSTEM_NAME=${CMAKE_SYSTEM_NAME})
 add_RunCMake_test(GoogleTest) # Note: does not actually depend on Google Test
 add_RunCMake_test(TargetPropertyGeneratorExpressions)

Tests/RunCMake/GetPrerequisites/RunCMakeTest.cmake

@@ -0,0 +1,3 @@
+include(RunCMake)
+
+run_cmake_command(TargetMissing ${CMAKE_COMMAND} -P ${RunCMake_SOURCE_DIR}/TargetMissing.cmake)

Tests/RunCMake/GetPrerequisites/TargetMissing-stderr.txt

@@ -0,0 +1,3 @@
+^warning: target 'does_not_exist' is not absolute\.\.\.
+warning: target 'does_not_exist' does not exist\.\.\.
+result_var='value;before;call'$

Tests/RunCMake/GetPrerequisites/TargetMissing.cmake

@@ -0,0 +1,4 @@
+include(GetPrerequisites)
+set(result_var value before call)
+get_prerequisites(does_not_exist result_var 0 0 "" "")
+message("result_var='${result_var}'")