Fix a bug where null would be converted to "null"

It's unclear what the concrete effects of this are, but they are not
very strong. This only happens in corner cases, when the input
connection is not active - while rotating, for example.

Change-Id: I1d22459a6e94a8ecccb53cfcbc2d301b1d502204

java/src/com/android/inputmethod/latin/RichInputConnection.java

@@ -233,8 +233,10 @@ public final class RichInputConnection {
         // getCapsMode should be updated to be able to return a "not enough info" result so that
         // we can get more context only when needed.
         if (TextUtils.isEmpty(mCommittedTextBeforeComposingText) && 0 != mExpectedCursorPosition) {
-            mCommittedTextBeforeComposingText.append(
-                    getTextBeforeCursor(DEFAULT_TEXT_CACHE_SIZE, 0));
+            final CharSequence textBeforeCursor = getTextBeforeCursor(DEFAULT_TEXT_CACHE_SIZE, 0);
+            if (!TextUtils.isEmpty(textBeforeCursor)) {
+                mCommittedTextBeforeComposingText.append(textBeforeCursor);
+            }
         }
         // This never calls InputConnection#getCapsMode - in fact, it's a static method that
         // never blocks or initiates IPC.