Return "" not "null"

Some models such as LDU, WIFI do not have "gsm.version.baseband",
which should return "null" when it is called.
However it returns "" not "null"

Bug: 120685535
Test: Self-Test
Change-Id: Idc1363d45a0ece3a35f46da287e21015ac1387af

core/java/android/os/Build.java

@@ -1113,7 +1113,8 @@ public class Build {
      * null (if, for instance, the radio is not currently on).
      */
     public static String getRadioVersion() {
-        return SystemProperties.get(TelephonyProperties.PROPERTY_BASEBAND_VERSION, null);
+        String propVal = SystemProperties.get(TelephonyProperties.PROPERTY_BASEBAND_VERSION);
+        return TextUtils.isEmpty(propVal) ? null : propVal;
     }
 
     private static String getString(String property) {