Fix a null pointer exception in DumpRenderTree (issue 1792567)

The first test causes NullPointerException in DumpRenderTree because dumpAsText
could return a null string.

tests/DumpRenderTree/src/com/android/dumprendertree/TestShellActivity.java

@@ -165,7 +165,8 @@ public class TestShellActivity extends Activity implements LayoutTestController
             if (mDialogStrings != null)
                 os.write(mDialogStrings.toString().getBytes());
             mDialogStrings = null;
-            os.write(webkitData.getBytes());
+            if (webkitData != null)
+                os.write(webkitData.getBytes());
             os.flush();
             os.close();
         } catch (IOException ex) {