Allow to remove characters

まだ誰も着手していません。

### Summary
I am trying to remove these characters - `(Primary)` from the `Find` field, by not adding anything in the `Replace` field

### Outcome
What ends up happening is the plugin wants to replace it with `()`

### Expectation
I'd like for it to replace it with no characters, essentially deleting them.

See my screenshot for reference:

Thanks!

Start by tracing the plugin's Find and Replace field handling, especially how an empty Replace value is interpreted. Reproduce the `(Primary)` case shown in the issue and verify that an empty replacement removes the matched characters without adding parentheses.

索引モデルが issue の本文から書いたものです。