[wpilibc] Return empty string instead of null for GetOpMode (#8754)

Clang 21 catches returning `0` from `GetOpMode` as returning `null`.
Since this state is very momentary and all comparisons inside WPILib are
done against `GetOpModeId` instead, I changed it to return an empty
string.

wpilibc/src/main/native/cpp/driverstation/DriverStation.cpp

@@ -695,7 +695,7 @@ int64_t DriverStation::GetOpModeId() {
 
 std::string DriverStation::GetOpMode() {
   if (!::GetInstance().userProgramStarted) {
-    return 0;
+    return "";
   }
 
   return GetInstance().OpModeToString(GetOpModeId());