fix compiler warning about varags and null pointers

Change-Id: I9faa505d30830ee168c9b61f3aaab406b8fd0c42
diff --git a/core/api/src/test/java/org/onosproject/ui/chart/ChartModelTest.java b/core/api/src/test/java/org/onosproject/ui/chart/ChartModelTest.java
index df2e91d..d55434b 100644
--- a/core/api/src/test/java/org/onosproject/ui/chart/ChartModelTest.java
+++ b/core/api/src/test/java/org/onosproject/ui/chart/ChartModelTest.java
@@ -36,7 +36,7 @@
 
     @Test(expected = NullPointerException.class)
     public void guardAgainstNullSeries() {
-        cm = new ChartModel(null);
+        cm = new ChartModel((String[]) null);
     }
 
     @Test
diff --git a/core/api/src/test/java/org/onosproject/ui/table/TableModelTest.java b/core/api/src/test/java/org/onosproject/ui/table/TableModelTest.java
index ac30391..b8ca57f 100644
--- a/core/api/src/test/java/org/onosproject/ui/table/TableModelTest.java
+++ b/core/api/src/test/java/org/onosproject/ui/table/TableModelTest.java
@@ -57,7 +57,7 @@
 
     @Test(expected = NullPointerException.class)
     public void guardAgainstNull() {
-        tm = new TableModel(null);
+        tm = new TableModel((String[]) null);
     }
 
     @Test(expected = IllegalArgumentException.class)