compose box: Fix a nullability nit in type signature.

The logic here is already correct, with a condition just under this;
just need to make the signature agree.

This issue is found by Flow when we unify the ComposeBox files,
coming soon -- Flow unfortunately doesn't check *.android.js.

src/compose/ComposeBox.android.js

@@ -74,7 +74,7 @@ type State = {
   selection: InputSelectionType,
 };
 
-export const updateTextInput = (textInput: TextInput, text: string): void => {
+export const updateTextInput = (textInput: ?TextInput, text: string): void => {
   if (!textInput) {
     // Depending on the lifecycle events this function is called from,
     // this might not be set yet.