Fix crash on shift with empty keys

Tapping shift might call `Utils.capitalize_string` on some symbols
(notably custom keys), which crashes on empty string.

This also happens on builtin layouts with `key1="\"`.

srcs/juloo.keyboard2/Utils.java

@@ -12,6 +12,8 @@ class Utils
   /** Turn the first letter of a string uppercase. */
   public static String capitalize_string(String s)
   {
+    if (s.length() < 1)
+      return s;
     // Make sure not to cut a code point in half
     int i = s.offsetByCodePoints(0, 1);
     return s.substring(0, i).toUpperCase() + s.substring(i);