Fix dump-tzdata.sh for "zic -b slim" files am: de11b51fbc

Original change: https://android-review.googlesource.com/c/platform/system/timezone/+/1454836

Change-Id: I4bf8bfad7bf0944c2fbee145829f95b948f236a2
diff --git a/debug_tools/host/main/java/TzFileDumper.java b/debug_tools/host/main/java/TzFileDumper.java
index d06a66c..5979328 100644
--- a/debug_tools/host/main/java/TzFileDumper.java
+++ b/debug_tools/host/main/java/TzFileDumper.java
@@ -285,8 +285,8 @@
             Object[] row = new Object[] {
                     type.gmtOffsetSeconds,
                     type.isDst,
-                    type.ttisgmt,
-                    type.ttisstd,
+                    nullToEmptyString(type.ttisgmt),
+                    nullToEmptyString(type.ttisstd),
                     formatDurationSeconds(type.gmtOffsetSeconds),
                     formatIsDst(type.isDst),
             };
@@ -299,6 +299,10 @@
                 "[gmtOffset ISO]", "[DST?]");
     }
 
+    private static Object nullToEmptyString(Object object) {
+        return object == null ? "" : object;
+    }
+
     private static void fillIntArray(MappedByteBuffer mappedByteBuffer, int[] toFill) {
         for (int i = 0; i < toFill.length; i++) {
             toFill[i] = mappedByteBuffer.getInt();