[SYCL][E2E] Fix Config/select_device.cpp when using GCC 15 (#18575)

The allowList can be empty when performing the check on the line
changed. This is usually not an issue, as it ends up evaluating to
false, however when using GCC 15, and compiling with `-O0` this will
crash instead. Adding the empty check to avoid this crash.

Author: ayylol

sycl/test-e2e/Config/select_device.cpp

@@ -111,7 +111,7 @@ static std::vector<DevDescT> getAllowListDesc(std::string_view allowList) {
     auto result =  allowList.substr(0, pattern_end);
     allowList.remove_prefix(pattern_end + 2);
 
-    if (allowList[0] == ',')
+    if (!allowList.empty() && allowList[0] == ',')
       allowList.remove_prefix(1);
     return {std::string{result}};
   };