Do not die when printing empty prefs.

Addresses issue:
http://buganizer/issue?id=1974140

Change-Id: I0001d69e8d278d4ca900f5ce56ad50c0b4b6b694
diff --git a/core/java/android/preference/Preference.java b/core/java/android/preference/Preference.java
index fc39573..08a2a9f 100644
--- a/core/java/android/preference/Preference.java
+++ b/core/java/android/preference/Preference.java
@@ -1456,8 +1456,10 @@
         if (!TextUtils.isEmpty(summary)) {
             sb.append(summary).append(' ');
         }
-        // Drop the last space
-        sb.setLength(sb.length() - 1);
+        if (sb.length() > 0) {
+            // Drop the last space
+            sb.setLength(sb.length() - 1);
+        }
         return sb;
     }