fix(NewMessageNewFileDialog) - fix creation of file from Blank template

[Antreesy]

☑️ Resolves

• Fix File doesn't added to conversation, when create from attachments with 'Blank' or no template #9779
• Regression from 5ae4c82 - joining of empty template with others into one array
• this.selectedTemplate?.filename checks for the path to file, for example, Templates/Filename.md. With /Blank file, we receive a search error, but file will still be created in Talk folder

To reproduce:

1. in Files, click "+" => "Set Templates folder" (could be wrong with button title)
2. Add some templates from there
3. in Talk, click 📎 => "New text file"
4. Try to create a file with Blank or any of the existing templates
   
5. Everything should work
   5.1) Or use only steps 3-4, template is Blank by default

🚧 Tasks

• Code review

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not required
• 📘 API documentation in docs/ has been updated or is not required
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required
• 🔖 Capability is added or not needed

[Antreesy]

/backport to stable27

[ShGKme]

Works.

Though, I didn't understand what was actually changed in the mentioned commit, what caused regression...

[Antreesy]

Changed code based on suggestion, tested, still works