MetricId: Avoid byte[] and string ambiguity

Passing null to the function will result in conflict with Bluetooth proto
(int int int byte[] int)
Bug: 122571213
Test: Build

Change-Id: Idca767ca1797a24fff49c8caab55291ef83f0b67
diff --git a/tests/CarDeveloperOptions/src/com/android/car/developeroptions/core/instrumentation/StatsLogWriter.java b/tests/CarDeveloperOptions/src/com/android/car/developeroptions/core/instrumentation/StatsLogWriter.java
index fa08b63..61a1f44 100644
--- a/tests/CarDeveloperOptions/src/com/android/car/developeroptions/core/instrumentation/StatsLogWriter.java
+++ b/tests/CarDeveloperOptions/src/com/android/car/developeroptions/core/instrumentation/StatsLogWriter.java
@@ -31,7 +31,7 @@
                 attribution, /* from pageId */
                 SettingsEnums.PAGE_VISIBLE /* action */,
                 pageId, /* target pageId */
-                null /* changedPreferenceKey */,
+                "" /* changedPreferenceKey */,
                 latency /* changedPreferenceIntValue */);
     }
 
@@ -41,7 +41,7 @@
                 SettingsEnums.PAGE_UNKNOWN /* attribution */,
                 SettingsEnums.PAGE_HIDE /* action */,
                 pageId,
-                null /* changedPreferenceKey */,
+                "" /* changedPreferenceKey */,
                 visibleTime /* changedPreferenceIntValue */);
     }