AK: Use default constructor of Optional if an unset bit is not found

Author: https://github.com/supercomputer7
Commit: 0433c0780d
Pull-request: https://github.com/SerenityOS/serenity/pull/1366
Reviewed-by: https://github.com/awesomekling

AK/Bitmap.h

@@ -156,7 +156,7 @@ public:
             if (!get(j))
                 return j;
 
-        return -1;
+        return {};
     }
 
     Optional<size_t> find_longest_range_of_unset_bits(size_t max_length, size_t& found_range_size) const