ExternalProject: Fix env mods with empty command

Fixes a bug when working with external projects with empty commands and
environment modifications. This change ensures that we are only applying
environment modifications when there is a command to apply the
modification to.

Fixes: #27113

Modules/ExternalProject.cmake

@@ -2306,15 +2306,18 @@ function(ExternalProject_Add_Step name step)
     list(JOIN environment ";--modify;" environment)
     list(PREPEND environment "--modify")
 
-    set(_result "")
-    list(APPEND _result "${CMAKE_COMMAND}" -E env ${environment} --)
-    foreach(element IN LISTS command)
-      list(APPEND _result "${element}")
-      if("${element}" STREQUAL COMMAND)
-        list(APPEND _result "${CMAKE_COMMAND}" -E env ${environment} --)
+    string(REPLACE ";" "\;" command "${command}")
+    string(REPLACE "COMMAND" ";" command "${command}")
+
+    set(__cmd)
+    foreach(__item IN LISTS command)
+      list(APPEND __cmd "COMMAND")
+      if(__item AND NOT "x${__item}" STREQUAL "x;;")
+        list(APPEND __cmd "${CMAKE_COMMAND}" -E env ${environment} --)
       endif()
+      list(APPEND __cmd "${__item}")
     endforeach()
-    set(command ${_result})
+    set(command ${__cmd})
   endif()
 
   # Replace location tags.

Tests/RunCMake/ExternalProject/EnvVars.cmake

@@ -11,8 +11,10 @@ ExternalProject_Add(CustomCommandEnvVars
   UPDATE_COMMAND ""
   PATCH_COMMAND ""
   LIST_SEPARATOR ,
-  CONFIGURE_COMMAND "${CMAKE_COMMAND}" -P ${ScriptPath}
+  CONFIGURE_COMMAND ""
+  COMMAND "${CMAKE_COMMAND}" -P ${ScriptPath}
   COMMAND "${CMAKE_COMMAND}" -DVARNAME=Stage -P ${ScriptPath}
+  COMMAND ""
   COMMAND "${CMAKE_COMMAND}" -DVARNAME=ListVar -P ${ScriptPath}
   CONFIGURE_ENVIRONMENT_MODIFICATION
     Stage=set:config