increase test coverage for libopendrive converter test

1. add test for the default condition of GetContactType() function, in order to do this, move two functions to separate file "libopendrive_converter_helper.h"
2. add test for the "return false" condition for the Convert() function
3. modify some original code: check vector size before calling

RoadLogicSuite/Internal/BUILD

@@ -73,6 +73,7 @@ cc_library(
     visibility = ["//RoadLogicSuite:__subpackages__"],
     deps = [
         ":libopendrive_road_object_converter",
+        ":libopendrive_converter_helpers",
         ":open_drive_data",
         "//RoadLogicSuite/Internal/Geometry:arc",
         "//RoadLogicSuite/Internal/Geometry:line",
@@ -97,6 +98,27 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "libopendrive_converter_helpers",
+    srcs = ["libopendrive_converter_helpers.cpp"],
+    hdrs = ["libopendrive_converter_helpers.h"],
+    visibility = ["//RoadLogicSuite/Internal:__subpackages__"],
+    deps = [
+        ":open_drive_data",
+        "@libOpenDRIVE",
+    ],
+)
+
+cc_test(
+    name = "libopendrive_converter_helpers_test",
+    timeout = "short",
+    srcs = ["libopendrive_converter_helpers_test.cpp"],
+    deps = [
+        ":libopendrive_converter_helpers",
+        "@googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "libopendrive_road_object_converter",
     srcs = ["libopendrive_road_object_converter.cpp"],