Fix broken OPTIX_ROOT_DIR environment variable for setting up build

Changes in 42713bf made it ignore the environment variable if there is a
defined but empty CMake variable. Also make similar changes for other
GPU compute APIs, to guard against future problems like this.

Pull Request: https://projects.blender.org/blender/blender/pulls/111928

build_files/cmake/Modules/FindHIP.cmake

@@ -8,7 +8,7 @@
 #  HIP_FOUND, if the HIP toolkit is found.
 
 # If `HIP_ROOT_DIR` was defined in the environment, use it.
-if(DEFINED HIP_ROOT_DIR)
+if(DEFINED HIP_ROOT_DIR AND HIP_ROOT_DIR)
   # Pass.
 elseif(DEFINED ENV{HIP_ROOT_DIR})
   set(HIP_ROOT_DIR $ENV{HIP_ROOT_DIR})

build_files/cmake/Modules/FindHIPRT.cmake

@@ -8,7 +8,7 @@
 #   HIPRT_FOUND, if SDK found
 
 # If `HIPRT_ROOT_DIR` was defined in the environment, use it.
-if(DEFINED HIPRT_ROOT_DIR)
+if(DEFINED HIPRT_ROOT_DIR AND HIPRT_ROOT_DIR)
   # Pass.
 elseif(DEFINED ENV{HIPRT_ROOT_DIR})
   set(HIPRT_ROOT_DIR $ENV{HIPRT_ROOT_DIR})

build_files/cmake/Modules/FindOptiX.cmake

@@ -12,7 +12,7 @@
 #  OPTIX_FOUND, If false, do not try to use OptiX.
 
 # If `OPTIX_ROOT_DIR` was defined in the environment, use it.
-if(DEFINED OPTIX_ROOT_DIR)
+if(DEFINED OPTIX_ROOT_DIR AND OPTIX_ROOT_DIR)
   # Pass.
 elseif(DEFINED ENV{OPTIX_ROOT_DIR})
   set(OPTIX_ROOT_DIR $ENV{OPTIX_ROOT_DIR})

build_files/cmake/Modules/FindSYCL.cmake

@@ -12,8 +12,12 @@
 #                 This can also be an environment variable.
 #  SYCL_FOUND, If false, then don't try to use SYCL.
 
-if(NOT SYCL_ROOT_DIR AND NOT $ENV{SYCL_ROOT_DIR} STREQUAL "")
+if(DEFINED SYCL_ROOT_DIR AND SYCL_ROOT_DIR)
+  # Pass.
+elseif(DEFINED ENV{SYCL_ROOT_DIR} AND NOT $ENV{SYCL_ROOT_DIR} STREQUAL "")
   set(SYCL_ROOT_DIR $ENV{SYCL_ROOT_DIR})
+else()
+  set(SYCL_ROOT_DIR "")
 endif()
 
 set(_sycl_search_dirs