notify the user that we only overwrite text with length >=2

### ⚠️ This issue respects the following points: ⚠️

- [X] This is a **bug**. Not a [question or feature request](https://github.com/gsantner/markor/issues/new/choose).
- [X] The topic is **not** already reported at [Issues](https://github.com/gsantner/markor/issues?q=#js-issues-search). _(I've searched it)_.
- [X] Markor **is** up to date. See [Releases](https://github.com/gsantner/markor/tags) for the latest version. Updates are available from [F-Droid](https://f-droid.org/en/packages/net.gsantner.markor/) and GitHub.
- [X] The bug is still present in the latest development version (git master). (Please [download](https://nightly.link/gsantner/markor/workflows/build-android-project/master) and try the test version of Markor, named **Marder**. Don't worry; Markor and Marder appear as completely separate applications. You can install both side-by-side, and Markor settings are not touched. In case the issue is resolved there, you don't need to create a bug report. The change will be part of the next Markor update.)

### Description

I created a file named "A.md" with content "A". Then I click FAB to create "A.md" again. Markor doesn't redirect" to the existing file and just creates a new A.md with empty content.
It only happens when the length of the content equals 1.

### Steps to reproduce

1. Create a file named "A.md" with content "A"
2. Back to file list
3. Click FAB to create a file with the same name "A.md"
4. See the error: markor shows empty content rather than redirect to the existing file

### Information

Android version: 8
Device:
App Version: 2.11.1

### Source

F-Droid

### Format / File type

Not specific

### Additional info / Log

```shell
- a relevant bug report https://github.com/gsantner/markor/issues/994
```

Reproduce the steps with an existing A.md containing the one-character text "A", then compare the behavior with the related issue #994. Trace the file-creation flow from the FAB and verify that creating the same name redirects to the existing file rather than showing empty content when the text length is 1.

Written by the indexing model from the issue text.