Conditionalize the "--modify" parameter on whether the list is empty.

cmake/modules/FindSwiftXCTest.cmake

@@ -125,8 +125,8 @@ function(add_swift_xctest test_target testee)
       OUTPUT ${test_main}
       # If the executable target depends on DLLs their directories need to be injected into the PATH
       # or they won't be found and the target will fail to run, so invoke it through cmake.  Because
-      COMMAND
-        ${CMAKE_COMMAND} -E env --modify
+      COMMAND ${CMAKE_COMMAND} -E env
+        $<$<LIST:LENGTH,$<TARGET_RUNTIME_DLL_DIRS:GenerateXCTestMain>>:--modify>
         "$<LIST:TRANSFORM,$<TARGET_RUNTIME_DLL_DIRS:GenerateXCTestMain>,PREPEND,PATH=>"
         --
         $<TARGET_FILE:GenerateXCTestMain> -o ${test_main} ${sources}