Fix NPE

Test: run ` atest DislayTest` on phone device
Fixes: 190400220
Change-Id: I7611bf499390beb32cfa5d73ae84c0e9a2cb638d
diff --git a/tests/tests/display/src/android/display/cts/DisplayTest.java b/tests/tests/display/src/android/display/cts/DisplayTest.java
index 0a702ab..dc44bf4 100644
--- a/tests/tests/display/src/android/display/cts/DisplayTest.java
+++ b/tests/tests/display/src/android/display/cts/DisplayTest.java
@@ -1103,7 +1103,7 @@
 
     private Point getVendorDisplaySize() {
         String value = PropertyUtil.getProperty("vendor.display-size");
-        if (value.isEmpty()) {
+        if (TextUtils.isEmpty(value)) {
             return null;
         }